Your symptoms point to you having perhaps an incompatibility with your Meshtastic board and the version of the Android app.
Meshtastic has gone through a lot of development in the last two years and some of the changes were breaking changes e.g.. Would be worth trying a flash and erase all (because the old config will probably confuse the heck out of the new application) on the iffy unit, and see if the Android MT app will recognise it.
You may then have the problem that the old version of meshtastic on the unfashed board won’t talk to the new flashed one so you’ll have to do the same on the other unit to get them to interwork.
May be useful to post the versions of the app and the firmware you’re running (usually visible on the OLED screen)