I finally got my MMS issue resolved. Maybe this will work for you. I switched from Google Fi to Xfinity. When I started to use my iPhone on Google Fi, they sent me a set of instructions with the SIM for how to set up your Cellular Data Network settings. See photo attached showing how I had to set up Cellular Data on Google Fi network. I had to erase all these settings to get MMS working with Xfinity mobile.
I knew my problem must be related to those settings. I assumed these settings would be erased when I used the Reset Network Settings option on the iPhone. They weren't. So I erased my iPhone back to a factory state, but when I reloaded everything from my iCloud backup it copied all the old Google Fi cellular network settings back. Still could not send MMS over SMS.
With the Xfinity SIM in my phone, the Cellular > Cellular Data Network screen is not available, so I couldn't go in and manually delete these settings. So, I put my old Google Fi SIM back in the phone so that I could see the Cellular > Cellular Data Network screen and I manually deleted the settings for Cellular Data APN, LTE Setup APN, MMS APN, and MMSC. Then I took the Google Fi SIM out of the phone, put the Xfinity SIM back in, performed a Reset Network Settings and then MMS was working fine on iMessage and on SMS. Problem solved.
So I would recommend you put the SIM back in your phone from your previous carrier, then check the settings on the Cellular > Cellular Data Network screen. Delete whatever is there. This is the only way to erase the Cellular Data and MMS settings from your previous carrier. If you don't have the SIM, you could try erasing the phone back to factory defaults but don't reload your stuff from a backup. You would have to install all your apps again manually.
Hope this works for you.
PS. I also had a good experience with Xfinity mobile tech support, but unfortunately they couldn't solve my problem. I finally figured it out on my own. 
