I can tell you, that I am pretty familiar with similar situation. In my case we decided just to change the Sim card, and RUT become working perfect.
The router were not working only with exact sim card form operator. With all other cards it were working perfectly. We got the answer, that the exact SIM card (inserted in second slot) is not allowing to send the data, and at the same time, this is blocking Sim card in first slot.
So we discussed with operator, they gave us different card, and everything started working.
We saw, that it was not a Teltonika RUT problem, it was because of bugging sim card.