Xfinity Never disclosed that Sending Pictures SMS or MMS on WIFI only, does NOT function.

I have to switch from WIFI to Mobile Data, to send pictures. You must physically shut WIFI off and turn on Mobile Data, then reverse the process if you don't want to use up Mobile Data.

I have had verizon and AT&T phones and while I'm at home connected to comcast wifi I can never send picture messages. I always have to shut off wifi and use my data. I've called them and they say no no its the phone providers fault.

My wife and I each have a Pixel 3a and both my sons have iPhones.  We our on XM as is one son, the other is on AT&T.  We leave data and wifi on all the time and are on wifi when we are home.  We are able to send and receive pictures and MMS messages without ever touching our settings.  I don't know if the pictures are transmitted via data or wifi and I guess I don't really care.

I was told that you can send text/picture SMS and MMS on WIFI only if you have WI-FI calling and chat feature enabled on your phone.

We use Google Messages and have chat features (i.e., RCS) enabled.  What we have discovered though is if our sons send texts when our phones are off, they never get delivered.  If they are in airplane mode, they do.  Still haven't figured that one out.

I guess I  don't understand the issue. It was my impression that SMS and MMS are sent using the cellular network. I  send and receive SMS and MMS  using Xfinity Mobile Android phone all the time without  a problem. I don't have WIFI calling enabled and try to never turn data on . Only problem I have ever had was if I attach huge files  sometimes it takes a while to send them or if they are over the limit I reduce the file size and then VOILA!

This is my test to see if MMS uses data to send pictures:  with WIFI calling and chat feature turned on, I disabled data on my Android phone and sent a MMS to another Android phones and iphones.  The Android phones got my MMS message but not the iphones.  When I sent it to iphones my phone asks to turn on data first.  So I think MMS does not use data if sending between Android phones using Google message app with WIFI calling and chat feature on.

My friends use imessage app on his iphone and MMS does not use  data if sending between iphones.  When he sent a MMS message to my Android phone it used data.

1.) SMS and MMS use cell towers (or indirectly through WiFi Calling) to route text messages and use carrier servers. Cellular data must be turned on for MMS though many carriers don’t charge for cellular data used. XM charges for cellular data used. SMS and MMS do not use WiFi per se. 
2.) Advanced messaging such as iMessage, Google messaging, RCS, Verizon Message+, and others route text messages through Apple, Google, Verizon, and other servers. They use WiFi or cellular data if you are not connected to WiFi. Advanced messaging shows up on your billing as general data when you are not on WiFi, not by message. 

My understanding is that if chat features (i.e., RCS or Rich Communication Services) are enabled with Google Messages, then all messages use wifi if you are connected to wifi and cellular data if you are not.  RCS messages between Android users are stored on Google's servers (not Xfinity/Verizon) until they are delivered.  This is more like WhatsApp and it doesn't have the limitations of traditional MMS messages.  Since Apple uses a different protocol, messages sent from Android to Apple must be converted to traditional MMS and must use a cellular data connection, not wifi.  WhatsApp is probably a better way to communicate between Android and Apple devices if pictures are involved.