How many times are you going to have this issue raised by numerous people (over at least the past 8 months) yet continue to not fix it? Have tried all of your 'trouble shooting' and have spoken to your Cx service people (who told me just to use textra). I'm paying for a service which includes SMS and mms yet im told to use another app because you aren't providing the service I'm paying for.
The troubleshooting advice you've been given is a bit concerning. That is not the sort of solution we'd expect our agents to suggest.
I'm sure it's been suggested in the past, but a Network Reset tends to do the trick. Instructions may differ from handset to handset.
Can I get you to try the below?
Tap Settings > General > Reset > Reset Network Settings.
Settings → General → Reset → Reset Network settings.