Assumed clock settings (master, slave) are ok...
Did you check ADAT cables already with the mentioned loopback test by connecting ADAT OUT with ADAT IN on either Audient or ADI-2?
If there is no cable problem, then it is most likely that you don't route audio to the ADAT outputs of the Audient.
You can check this for example by watching the Global Level Meters of your ADI-2 Pro.
If there is no input signal ... nothing can be played in ADDA mode.
The routing in ADDA mode you can see in the block diagram of this mode, take a look, easy to understand.
Then you know whats going on in this routing mode.
The ADI-2 Pro supports SPDIF and even ADAT (takes the first two channels), thus wrong format over TOSLINK can be excluded.
If you are uncertain whether you changed already too much on the ADI-2 Pro, then I would do the following as default action
1. check / update firmware
2. check / update driver (although you run in adda mode, not USB, USB is only for firmware updates and remote control)=
3. perform a full reset of the ADI-2 Pro
4. save these default settings to user setup 9
5. configure ADDA mode, save this as user setup 1 "ADDA"
6. customize ADDA mode (clock slave, etc), save this as user setup 2 and stay in this setting to customize further
Now you can restore the default settings at any time from user profile 9.
Alternatively from user profile 1 as this is default plus ADDA mode instead of AUTO mode.
If the Audient is too tricky, then take into consideration that RME has a better product offering.
You can check for CRC errors pver USB in the driver settings. In the driver settings you also see directly any issues in terms of digital input status (lock and sync), also good for loopback tests.
TotalMix FX shows you in the top rows input signals from the devices inputs.
In the middle row all audio coming from the PC over USB.
Routing is very easy by selecting one HW output in the bottom line and then pulling the fader up of either top or middle row.
This way you can create an individual submix for each of the outputs as needed.
This is a very logic operation and works reliably and also troubleshooting is much easier.
Because you see the input signals in the top and middle row and know upfront if they are there, no issues.
And then you have the great flexibility for routing for every HW output in the bottom row of TM FX.
Recommended for example either BBF Pro or even better UCX II.
Most flexible products UFX II and UFX III (with MADI if you need this).
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13