Do you still have the original install file?
If not you can find old versions of the .apk to download.

There's a good chance your phone and/or possibly the app updated itself and things are no longer compatible with each other.
If you get the latest app you might well be practically forced to update the Phantom firmware too.
Being on all the latest should work OK, but if you can get back to what was working before it could be worth aiming for first.

Note there are at least two versions of DJI app it was called 'DJIgo' and is now 'DJIgo4' - not sure which you need to be honest but probably the earlier one.