I had the same exact problem (Telekom Deutschland Multi-SIM in RUT955, installed in a motorhome).
After having upgraded to firmware version 7 the SIM connected to the mobile network, but I was unable to access the internet (no data transferred).
I tried everything possible for days, and ended up with re-installing firmware version 6.08.X (X can be 3 or 5, both work).
The only drawback with this version is that my cell phone (with the master SIM installed) only rings once whenever the router is connected to the internet (just logged in, no data transfer). Must be something with the Teltonika router as the phone rings normal when the exact same SIM is used in a mobile hotspot.
I solved this problem by adding a second profile to RUT955 that disconnects the SIM when I do not need a mobile connection (e.g., when in reach of a wifi network I can connect the router to or do not use the RV at all).
Teltonika suggested to wait for firmware version 7.01 which is meant to solve these problems.