APN is usually the correct answer.
Also, ensuring you have a data plan (and connected).
Also, ensuring you're not trying to send via WiFi.
Some may suggest Factory Reset. Possibly a good idea if the above don't work.
If it worked and then six months ago it suddenly stopped, the question is "What did you do six months ago to cause it to stop?" -- Did you install an app? Did you modify your phone? Did you drop it in the toilet? (The last one is mostly a joke).
Things don't "just stop working" -- Most of the time it's something done to the phone.
Name: T-Mobile US
MMS: proxy 188.8.131.52
APN type: default,supl,mms[all other fields left blank]
This works. YAYYYY