Some updates.
Tried everything and there were no solutions until recently.
Reached out to support and FP is looking more in depth into it and seems to be working on this within A15 as “they hope this will fix it”. nothing concrete though on A15.
Provider updated something on the 5G Network the other day because without any FP Updates suddenly the 5G network became almost unusable at all on my FP4, even without moving.
So I tried one thing again and I’ve found a fix at least on the A1 Telekom Network: turning off 5g .
Moreover, my battery lasts at least 20% longer and the whole phone seems to be less “stressed” as I notice.
Been around a lot and never again lost the connection so far. Speed wise the connection doesn’t appear to be any slower for daily usage.
So for me this is a nice fix.
Reported everything to support too and they use this information for the A15 development
Maybe it helps others too as a fix.