re: Textra - Can't retrieve MMS

I had this same problem, this fixed it for me.
Go to the phone's settings app (not Textra settings), more, mobile networks, access point names, tap on the phone that shows up. Copy down what they show in MMSC, MMS Proxy, and MMS Proxy Port. Then go back to Textra settings, MMS Settings, check Manually Configure APN, and enter what you wrote down. Hope this works