Hello,
Did the router send SMS previously?
Did you try a different SIM?
Do both RUT955s have matching settings in Network -> Mobile -> General and Network -> Interfaces -> [mob1s1a1]
Otherwise, please update your router to the latest firmware version.
You will most likely not be able to to update straight to the current 7.04, thus try the latest legacy 6.09.2 first.
Update with Keep settings disabled.
If the issue is present on the latest firmware, generate a troubleshoot file and attach it by editing your question.
Best regards,