I’m not certain this is specific to the PractiScore app for Android. Also not certain what does “the app shows offline” really means.
As for troubleshooting sync issues see the following article. It was written to troubleshoot sync issues on Chromebooks and other virtualized environments, but all the things also applicable to syncing regular tablets. I.e. you need to make sure they all on the same wifi and wifi allows them to see each other.
Syncing with PractiScore app for Android running on Chromebook or in Bluestacks
PS: the Android app has special handling for local hotspot syncing, like when running Android app on the phone with hotspot enabled. I have no experience with hotspot on iOS devices and don’t know if it doesn’t have the “client isolation mode” enabled, which would prevent devices to see each other. In any case that is not specific to Android app