Can no longer HOST zoom meetings from home - can't teach classes any more!

Hi. Husband has been teaching his university classes for weeks in our condo with no problem.  Suddenly since (Wed. Oct 28) he can no longer host zoom meetings to teach his class. The participants or single person attending the zoom meeting  gets choppy video and audio. 

All other internet functions in condo are normal. Streaming, TV, etc. Hosting zoom is the only issue.

It does NOT fix the problem to connect directly into the modem with an ethernet cable, going to airplane mode, taking all other devices off internet, or sitting right next to the modem. It does NOT fix it to use a different laptop to host the zoom meeting. 

A condo building nearby may have added Xfinity - the parking lot was filled with Xfinity trucks. Right about the same time the problems began.

I've asked other neighbors in the building if they are having issues, but haven't heard back yet.

The problem disappears if he goes to my daughter's home in another city. 

Xfinity is bundled into our condo fees as a bulk item. The modem is only a year old or so. Please help.