> When I awake the iPad it says connected, but after 5 secs this changes to offline.

What "says connected" ? Do you mean the iPADs WLAN connectivity ?
Or do you mean the TotalMix FX remote application ?

If you changed the DSL router it could be the case, that you get another DHCP pool and also other IPs.
Maybe the IP adress and port number of TotalMix on the Desktop PC does not fit anymore with the
IP address and port number, that you initially configured in the TotalMix Remote application on your iPAD.

First we need to ensure/check
- whether there is communication between the devices and
- that IP addresses do not change just in case you are using DHCP services which is much likely

For that I need to ask you a couple of question in regards to your network:

1. Whats the IP address, Netmask, default GW of Desktop PC, iPAD and new router ?

2. Are both, PC and iPAD, using Wireless LAN (WLAN) ? Or only the iPAD ?
    If both devices use WLAN, then you need to check whether WLAN devices are allowed to "see" /
    "communicate with" each other.

    This you can e.g. check by trying a ping the IP address of the iPAD from the Desktop PC.
    Alternatively check this on the DSL router itself.
    My Fritzbox has such an option under WLAN -> Security
    [x] Die unten angezeigten aktiven WLAN-Geräte dürfen untereinander kommunizieren
         translated: The active WLAN devices shown below may communicate with each other
    For other devices you might find this under a different place and using a different wording....

3. Which devices use DHCP or which use static configuration of IP address, netmask, default GW, DNS server ?

Can you successfully ping from Desktop PC to iPAD ?

From theory .. how it should work .. what you need to perform on the application:

a) Desktop PC - where the recording device is connected to
    If you enable Network Remote settings in TM FX, then it takes the IP address of the network adapter
    and - if you need - you can change the port number for this application, default: 7009
    On this side you can not do much wrong.
    You only need to ensure, that this PC always gets this IP address, either by static configuration
    or by configuring DHCP services in your DSL router, to give the DHCP clients always the same IP.
    This is usually not enabled by default, check this in the DHCP services of your DSL router.

b) On the TotalMix Remote client / iPAD:
    Ensure that this device is in the same LAN as your Desktop PC (or you need to set a proper default GW entry).
    Ensure that from iPAD you can ping the Desktop PC's IP address.
    If iPAD has no ping utility or if you do not find: then try it vice versa to ping the IP of iPAD from the PC.
    If ping does not work, then this indicates already a problem.
    Otherwise ... if the devices are in the same LAN and can ping each other, then there should be no issue.
    On iPAD in TotalMix FX Remote you only need to enter the proper IP address and port number of the Desktop PC.

From your description its still not clear to me whether TM Remote on the iPAD connects to the TM FX counterpart on the PC.
Maybe even works for a little while and then after a few seconds of operation fails or something like this.

You said:
> When I awake the iPad it says connected, but after 5 secs this changes to offline.

What means "this changes to offline" .. ? What do you mean with "this" and "offline"?

Could you perhaps post a picture or a short video what happens ?

To your question of port forwarding
and another idea for troubleshooting other potential issues that can happen and can lead to instability problems.

The configuration of port forwarding is only required if you want to access a device from your internal network from external (from internet). From your description the communication between Desktop PC and iPAD happens on the internal LAN. From internal LAN to (also internal) WLAN and vice versa. If both devices can ping each other, then you should theoretically have no issue with the IP communication between both devices (be it using tcp or udp).

But even if both devices can ping each other you might have other issues:

Did you check already for
- a bad network cable or
- speed/duplex Problems on the LAN connection between router and Desktop PC ?

Speed / Duplex problems are tricky. Traffic with only a few packets like ping goes through, but as soon as you have more traffic then you run into issues. https://en.wikipedia.org/wiki/Duplex_mismatch

And you are sure that in TM FX remote on the iPAD you configured the proper IP address and port number of Desktop PC and proper port where the application is listening to ? Could you kindly provide a screenshot of TM FX Remote configuration on PC side and iPAD side ?

And any impact of Firewall configured on the PC, that might block UDP, can be excluded ?

EDITED, sry pls read again.

According your assumption, correct, I have such a suspicion as well, the question is, by what this is caused.

If you could kindly check for speed duplex or cable problems on the network adapter of the PC, this would be still somethnig that you could / should check to exclude problems in this area.

The next thing would be a sniffer trace on the Desktop PC using "Wireshark" to check whether application traffic from the iPAD arrives on the Desktop PC. This is theoretically not too hard and could eventually be done by yourself.
At least you could try.

If there is no communcation this would be bad, then somewhere the traffic is blocked (-> Firewall, Windows / iPAD ?) or you are using bad IP addresses (configuration mistake).

If there is communication, then somebody would need to analyse whether the communication looks fine between those two devices.

BTW, I re-edited my post #9 ...
But as you repeat telling, you simply exchanged the router, etc etc ...

It could still be the case that you use DHCP and that both devices got different IP addresses now.

Now as you looked at the IP address of both devices and performed ping .. you did it now with proper IPs, as you just looked them up.

I have still the suspicion, that you have still an old / invalid IP address configured in TM Remote on the iPAD.
If your devices got other IP addresses by DHCP now, then you need to change the configuration on the iPAD and configure the new proper IP addresses of your PC .... has this been checked by you ?
I assume the iPAD still tries to connect to the old IP of the Desktop PC but this IP changed now (most likely) ! Pls Check.

EDITED:

You never mentioned explicitely the IPs and netmasks that are configured on both devices, so it was not possible to tell explicitely whether there is a configuration problem . I simply have to assume this now.

A screenshot of the IP/netmasks of both devices (PC, iPAD) would be helpful and
a screenshot of the IP configuration in TM Remote on the iPAD.

You need to check whether the applications on iPAD and PC communicate over network.

I would install Wireshark on the PC and take a sniffertrace to check that: https://www.wireshark.org/

You know source and destination ip address and the protocol number 7009.
By this you can filter in Wireshark for this network communication.

Tip: do not use "promiscuous mode" to see only the interesting traffic from/to your machine.

Good luck with that.

P.S.: I am not sure whether you feel comforatbly with such things and support only via forum has its limitations.
The best might be to contact official RME support to work on that and to align on further actions.

P.S.: you should also ensure, that you revert any NAT / port forwarding settings in the router in regards to port 7009 !!
NAT / port worwarding is not required in your setup and maybe this is causing issues.
This one could also see in a sniffer trace with wireshark (if there is no communication between iPAD and PC...
Traffic between iPAD and PC will definitively go via Router, because the router bridges LAN and WLAN.

Nice that you found it finally and many thanks for your verbose feedback how to solve it.
Surely also very helpful for others with similar issues.