Smart TV's don't retain connection to wifi (or to the router directly)

I've used Smart TV's for years with Xfinity internet and had no problems, but in the last 3 months or so I've had difficulty maintaining an internet connection to all three of them.  They behave the exact same way.  I'll be watching a show on one of my streaming services and it just loses connection.  Or, I'll have finished a recording and go to select something else to watch and all of a sudden I've got nothing.  I can restart the gateway, then restart the TV, then unplug the gateway, spin around three times, wait one minute, plug it back, then restart the TV, etc. - basically all of the recommendations for resolving the issue, and no amount of restarting, unplugging, etc. change the behavior.  I believe it might be a band issue.  I went into the Admin site and turned off the 2.4 ghz band and my TVs magically work consistently.  But, then my printers don't.  What do I need to do to allow all of my devices to connect and retain their connection?  We've got the XB8 gateway.