Check APN Settings
If you are not able to send even a single message SMS or MMS, then it could be a network issue. Check your APN settings. Search for your APN settings in our forums or on your favorite search engine.
Other things to check:
- SMS spam/block - make sure your friend/family is not blocked.
If you are using the default Messages app, then tap the 3 bars on top-left > “Spam & blocked”
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Enable the “Auto-download MMS when roaming” setting in Messages > top-right profile icon > Messages settings > Advanced (reference from TMobile forums)
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Try a different app like QkSMS (open source) to see if the issue persists