Cant join mediaplayers, how to troubleshoot?

Since recent times a lot has improved on joining media players, so i had good hopes it would now work. but, i still cant get it to work.

I have 2 wifi speakers from the brand JAM, model rhythm and symfony. In the JAM app i can easily combine/group the 2 speakers, but in HA i cant get it to work.

When i try joining it ‘seems’ to work, the moment i link them the speaker playing stops and it shows in my dashboard that they are now in a group of 2 speakers. But when i then press play again, only the original speaker starts playing (still showing a group of 2)

Is there a way i can get this to work, some tips/trick that i can try to trouble shoot?
I really want this to work because the JAM app is no longer updated, so that will stop working at some point.

This is the device page,showing hardware and firmware version. if i check the github page of linkplay this model of speakers is there and the fimware is newer then needed.