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!!!!
@user63J5YhnY6s i dont know if xfinity has this but on stock messaging app can you check the Message Center Number if ever xfinity has it and then check their website if it's the right number? It should start with "+" sign.