i also have a ja-mobil SIM (MNO: Telekom Deutschland) as daily driver on a 10III since jolla release sailfish for it.
Depending on there i am - ipv4 telegram(android)/fernschreiber are working.
At home i don’t have any connection at all.
When traveling i sometimes have network connection - it feels when connecting to 2G Networks due to bad coverage i get ipv4, sometimes it “stays” connected even when switching to 4G
When I was roaming in switzerland i had no problems - telegram for 3 days without wifi…
The situation is the same with or without VoLTE.
I also have a Lidl-connect (MNO: Vodafone) in SIM2, there i have a 100% success rate, network is working as expected everytime. Even if i switch the sims, the behavior is the same.
Both sims worked in a 10I like charm before switching to 10III.
I am also effected with the problem when switching to/off wifi and losing network especially in android, so i only switch it on when needed.