I had a similar problem when trying to SMS from my modem to activate a T-Mobile account online access. I found a topic that discussed upgrading the cellular modem firmware. This is not the same as the device firmware. I upgraded it, and now the SMS works. You have to refresh the page, however, to receive the SMS texts.
Discussion/process on upgrading the cellular modem firmware can be found here:
I’m having this issue with our BR1 Pro 5g. I’m running 8.5.0 and running cellular firmware T99W175.F0.1.0.0.9_063 (which is the latest). Trying to send an SMS gives me;