You need to „purchase“ (although its free) the Totalmix Remote App with your own appleid once on an iPad running at least iOS 12.
Looks like your friend has a newer one? If so - Ask him to login once with your appleid in App Store on his new iPad. Just to be able to get the app. You can logout afterwards, you can delete the app on his iPad afterwards.
Then, back on your legacy iPad, go to your account‘s „purchased“ apps and you’ll see the TM remote app. After hitting download, you’ll get a notification that you can only download the last compatible version… hit „OK“… Voila.
The „trick“ here is that the app has to be „purchased“/owned once via your appleid (or a family member-but that is another story).
Additionally it makes sense to use the latest TotalMix FX App v1.99 on your mac. Available as single file download in the RME downloads page. Select the software section on the right side there.
“Do It For Her”
My Gear: Bontempi Magic light Keyboard