Hi, Xperia 10 III user here. I am also having german ja-mobil (t-mobile/congstar) in germany. In Slot 1 i only get internet connection with “dual” IP does not connect - so same issue here. With “dual” VoLTE does work, but the hotspot does not.
Using Slot 1 hotspot is working, VoLTE did not work, and Internet connection sometimes works - not shure if dual or IP, testet both… Maybe IPv4 mobile data connection not possible - #56 by cartron does help, cause its related?!