I have an S20, my husband had and S8 and just upgraded to the S21 and is still having this issue. We can't send or receive pics via text message from each other unless we get off the wifi. Neither one of us has this problem with anyone else. It's not a home wifi issue as this happens anywhere we go. Anyone know how to fix this or what the issue may be? We have service through AT&T.
MMS is usually carrier related, so try talking to your carrier first and see what they say.
If they can't help... then recreate the issue and submit an issue report using the Samsung Members app right after it.