I don't think it's a carrier setting. I have two S10e's and they both show the same carrier info etc. One works fine, the other doesn't. The only difference is my phone (the one that has no problems) is using Google Messages. My wife's phone, the one that won't send MMS with WiFi on at all, is using the stock Samsung Messages app.
So after multiple attempts and contacting Xfinity, I reset carrier settings for the second or third time and now MMS will send over WiFi. It still takes between 45-75 seconds, but it does go through. Xfinity said if it continues to happen we'll need to change the SIM card.
I am having the same problem with my Samsung 10E. I Did everything you said and also changed the SIM card. It didnt help. Have been going back and forth with comcast for months now and no solution in site. Sometimes the pictures will send in a couple seconds and other times it'll take anywhere between 30 mins to 2 hours to send one picture. Most of the time it's the longest version. We also replaced our router twice. Reset our wifi at least 6 times. What I want is a new phone. I have only had this one for 4 months and the problem started 15 days after initial purchase. UGH!!!!
First - this is NOT an Xfinity problem. I had (yes, "had" - we'll get to that) the problem and it was an issue on any wifi connection - home (Xfinity), work (Verizon), my brother's house (Cox)... In addition, I have a Samsung S9+ and my wife has an S9. We got them at the same time. Hers has never had this problem. We both use Messages from Google LLC instead of Messages from Samsung, although I tried both and had the problem with both. Oddly, we have different versions of the Google app listed - hers is newer - so I was trying to get that one to load on my phone, with no luck. However, uninstall and reinstall of the Google Messages application resolved my issue. I'm not sure what setting I may have missed. I thought I had checked them all - even the advanced, and I tried the metered suggestion listed here which didn't work for me. So no smoking gun... just a possible solution for you.