My Conclusion
After fixing this issue, I tried to use mbim again. However, I was still experiencing some issues. I'm relying on Tailscale in my setup and as soon as I would do anything over this interface, the connection would just crap itself and only a restart of the mbim interface would fix the connection. I was unable to ping from any device to the internet from any interface. Same problem with qmi. Then, I switched back to modemmanager, which worked flawlessly. However, I was still experiencing issues with the modem. I was losing the connection every 5-60 minutes (similar to this issue). The openWRT side seemed fine but I was not getting any responses to pings. I unfortunately can't get into more detail here. Debugging the connection on the modem side is pretty hard. I think you can do it via the unsolicited result codes. However, I was having issues monitoring them while modemmanager was runnning. Running watchcat or luci-app-lite-watchdog in the case of mbim/qmi helped, but the interface restart would happen so often that the connection was basically unusable. I was also pretty tired of this anyway, so I just tried a different modem. Also tried rooter goldenorb but the flexibility of openWRT outweighs the convenience features. Not beeing able to install kmods (or having to compile them myself) was too tiring for me at this point.
With the Fibocom L850-GL the umbim solution wouldn't give me a IPv6...I suspect this is because this modem expects host SLAAC for IPv6, like described here. I didn't look further into this.
In the end, I switched to the Fibocom L850-GL and modemmanager. Forced disconnects from my ISP happen every 24h and I use watchcat to automatically restart the interface.
From my research it seems like there is still some work to be done with cellular connections in openWRT. Especially with the forced disconnects and IP changes. See the discussions and proposals here, here, here, here, here and here. Progress is beeing made.
I'm not really confident in saying this but I think using modemmager probably is the preferred solution at the moment. It also seems to be the project where support for all the different modems and all of their quirks is beeing worked on.
Some luci apps for modemmanager like the ones from 4iceG would also be nice....maybe I'll try to make one when I have more free time at some point in the future.
Will also try to make a PR for this change when I find time.