Guys,

RME addressed this matter some time ago.

  https://forum.rme-audio.de/viewtopic.php?id=28980

For me?  I've no desire for more "Creature Features", as I much prefer the Sterling Audio Qualities of the RME over any added fluff.  The existing remote is fine.  Granted, my old remote is small, and easily lost in Sofa cushions.  Occasionally, my Wife sits on it. But with only a simple Outpatient procedure to extract it, things are good!    Can you believe the Battery lasted 2 years?  After all that?   Oh well... If I need more?  Then I simply get off my dead a$$, and twiddle an encoder.  Good exercise!

Button pushing notwithstanding, Let's enjoy our RMEs for their intended purpose! 

Just sayin!

Curt

You can use every Logitech Harmony remote.

I use a Harmony Smart and an older model 900 (no longer available) with color display but also the entry level 350 works very well for my customers. The 900 with the touch display is great because you have access to all available IR codes or you define your own hotkeys.

It is difficult to recommend a specific model because it depends on your requirements, budget and taste.

There is a very cheap way— you can just get a ir transmitter wiring to a headphone jack. It probably costs within one dollar only. You insert it into your mobile phone. There are apps that can send those commands. You can also program a wav file pack  that encode those commands.

100% agree RME stuff is usually in thousands of euros range, don't get what difference 100-200 euros make for high end remote, and if it does you shouldn't heave spend so much (non disposable) cash on audio gear in the first place.
I have built my own remote with NovelKeys BOX Royal switches, it's big, dumb, and I love it! I did it mostly for practice and it's not very useful though.

There isn't much to it apart from novelty that it's using mechanical keyboard switches, and it's comically large, it's just an arduino template and custom pcb, I use it when I need functions that aren't on RME remote; I made it to practice soldering LOL.

Search google for "3.5mm Headphone IR Remote Control for Phones". sometimes you can find as low as $1.
It is also possible to do it DIY --- Just wire two IR LEDs (you can get solder it off from old IR controllers) to a 3.5mm headphone male connector and you are done..

Insert this into a Phone/PC with headphone jack.

There're various software available on iOS/Android/PC for you to save a list of general purposed control code, convert that to audio stream, and play it through the headphone jack. Getting the voltage signal, the IR LEDs will then emit corresponding signal to the ADI-2.

For Android a forum member actually have the custom settings ready for use:
https://www.forum.rme-audio.de/viewtopic.php?id=29169

On Linux standard LIRC library can take care this for you as well.

On PC/Mac there's a software program called IrScrutinizer which can not only send the signals, but also save the commands as audio wave files so you can essentially play it later without any special software.

This is fun to DIY, costs <$1, and works like a charm.

For me because I usually use my ADI-2 Pro with my own music player code (based on MPD). I have a set of key bindings to emit the recorded remote control audio stream to my computer's/Raspberry Pi's headphone jack. So I can control my music library / music playing / ADI-2 in the same software program with just one or two keystroke.