I will edit this post to reflect that
Looks like there is a time limit on edits, so will have to update in a new comment.
When GL.iNet staff logged in via Goodcloud they were able to run the Visible test script successfully. At any rate, while running the script the SMS function has stayed stable for 48 hours so far. So there seems to be hope for folks like me and @mdpatel.
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It’s not clear why the script worked for them and not me. The script was unchanged (same md5sum). The only difference I can see is they appear to have run it explicitly from the shell interpreter (sh auto_check_SMS_Visible.sh) instead of relying on the #!/bin/sh shebang line in the script. Since the script is not in $PATH I invoked it as ./auto_check_SMS_Visible.sh. I can’t think of a way it could make a difference but who knows. There only seems to be one sh on the system and it’s a link to busybox.