Thanks for taking the time to post to the forum. My apologies for the delayed response. Keep in mind that Advanced Messaging requires that both sender and recipient have Advanced Messaging installed on their handhelds for the service to work.
With that said, the xfinity wifi internet connection does not incorporate any restrictions that would prevent Advanced Messaging from functioning.
Let me know if you’ve noticed the issue to occur on any other wifi signals and also with recipients that also have Advanced Messaging. If you do notice the issue with other devices that have Advanced Messaging, turn wifi off and attempt to send the message through cellular signal.
Let me know what result you get.