We understand that we should have read more carefully about the dual-modem API/Forwarding functionality and also understand that this is your first router in which you introduced dual-sim support. Nevertheless, if you want to satifsy clients with SMS use-cases, you should provide functionality to treat the 2 modems separate and enhance the firmware accordingly.
We don't see why this would actually be complicated, as on the "Mobile Utilities -> Messages" screen it's possible to select the Primary or Secondary modem and see the SMS messages for the selected modem. So the functionality seems already there, it's just not exposed to the API.
We intend to buy more units from Teltonika in the future due to its good hardware quality, but SMS-support in multi-SIM devices such as RUTX12 needs to be improved accordingly.