If you have a thai Sim, and you have it with you, try calling your phone company such as True or AIS. Ask them if they can give you international service. I did it when I was in Thailand with True, they were glad to give me international service and 10,000 baht credit in case I need to use it extensively. I never had to use it, but that way when I traveled outside of Thailand, I would be able to use my Thai -number as normal even at a high cost per message and receive the OTP.
I had insurance from them, instead of the promised no claim discount I got 30% increase, although that’s not the agency’s fault. This is general medical insurance not just Covid.,