If the R21 comes up with lights (not just interior lights) and sounds as soon as it sees TIU power, it's missing the watchdog signal.
You say that other engines are able to be added to the app. If you have a DCS Remote, you can try adding the R21 to the remote. If it adds to the remote and works fine, I would suspect the app. If it doesn't add to the remote while your other engines do add, the engine's most likely the problem.
If you don't have a remote, then do a signal strength test using one of your other engines that did add to the app, and report what you learn.
