After restoring from lost IMEI problems with cellular network

After I tried to flash Oxygen OS some weeks back and loosing IMEI and WiFi with that, I was able to recover my IMEI and WiFi is working, too. However I have huge problems with the cellular network: I don't have any data transfer when I am not on HSDPA+, and the phone doesn't switch to GSM anymore.

Here's what I did:
1) Installed ColorOS using the help of the No IMEI/WIFI/BASEBAND? thread and got WiFi back
2) Flashed Kitkat (33R) with fastboot and got cellular network back (somehow)
3) OPO started to automatically update using OTA in several steps up to latest Lollipop with 230 modem - no more cellular network
The status bar shows the "SIM card missing" icon, switches to the empty triangle then to the filled triangle and immediately back to the "SIM card missing" icon in a constant loop.
4) Flashed 44S modem with TWRP 2.8.6.0 -> status as above: only data connection in HSDPA+ mode, no data connection in HSDPA and no connection at all otherwise

I am using the German E-Plus network.

I was trying all kinds of modems and tips. I tried the above procedure many times. This post is my last try ...

Any help is highly appreciated.