MMS cannot be sent when wi-fi is turned on for android phones when you have xfinity mobile. <Edited> force you to use more data, so you have to pay more.
For what it's worth, I have no issue whatsoever sending MMS messages while on Wifi, with my Samsung Galaxy S7 on Xfinity mobile. Do you have evidence that others are having the same issue as you?
It's just a recurring set of bug(s) - I see them too on Sammy S8, often. It is annoying but it is not a scam. Do not attribute to malice that which can be explained by ignorance.
Try the workaround ideas that have been posted here and keep posting your exact observations about the failure since that may help us all work-around the issue while the programmers try to fix it again, again, again, again.
One of the rather weak "workarounds" I sometimes use for the issue is to send the MMS via email over wifi instead. Can send it to the folks' email addresses or to their phones by using vtext.com.
so if you want to send a MMS with actual-text to phone number<Edited>, you can also send it as email - addressing it like so: "<Edited>" . (On xfinity mobile, this approach won't work for photos or videos, only pure text. On native verizon, it can work for photos via the vzwpix.com domain.)