ERROR 350 is one of those catch all errors which is not very deterministic as to where the issue might
So the above says you are on an M1 network, it is very hit and miss as to whether the connected network supports or will route SMS successfully. Couple of things to look at.
Assuming this is a Sierra SIM then check the portal out to see if you can see SMS’s have gone. If its not a Sierra SIM use the other operators portal to check to see if it made it to the core network. The +CMGS response oinly indicates that the layer 3 signalling completed, not that the SMS has gone through the whole system.
Push the unit back to 2G and send the SMS, if it does not work on 2G then you do have a problem.