XFinity (Comcast) and other cable Internet providers nowadays are providing their own hardware - usually combo units that are a cable modem (gateway), Wireless (WiFi) router, telephone adapter, and sometimes even cordless DECT phone base station as well. These combo-units are configured by your cable provider and usually have multiple wireless networks broadcasting.
The problem with these units is that the wireless functionality usually cannot be disabled by the subscriber. Calling your cable company can often lead to getting this disabled, but sometimes only after talking with multiple people. For this you would also need an RF meter to verify and make 100% certain that it actually went off. Lastly, the risk of going this route is that theoretically at any time the cable company could run a software/firmware or configuration update which resets your modem/router unit and the wireless is back on! You would never know unless you regularly checked with an RF meter to make sure the wireless is still off.
So, the best and safest solution is to replace these wireless devices with non-wireless devices. That is, replace your cable company's equipment with your own equipment that you can purchase. This equipment you purchase has no wireless functionality, so you never have to worry about it coming on - it's not possible for it to create RF radiation. A side benefit to this is that usually the cable company is charging you monthly for the equipment, and in a year or less you will be saving money every month by having purchased your own equipment and not paying that rental each month!
If you use cable Internet WITHOUT phone service, get a modem like one of these (in order of most expensive to least expensive):
Arris 32x8 Docsis 3.0 cable modem 1400 megabits/s speed: http://amzn.to/2ugC7Mk
Netgear 32x8 Docsis 3.0 cable modem 1400 megabits/s speed: http://amzn.to/2tcwhqc
Motorola 16x4 Docsis 3.0 cable modem 686 megabits/s speed: http://amzn.to/2sOb9Y4
TP-Link 24x4 Docsis 3.0 cable modem 1029 megabits/s speed: http://amzn.to/2sO5bGp
TP-Link 8x4 Docsis 3.0 cable modem 343 megabits/s speed: http://amzn.to/2tcBf69
Keep in mind most providers and most Internet plans are pretty slow - most do not even max-out the 343 megabit modems. But for a little bit more (like the TP-Link for 1029 megabits) you can future-proof your system a bit more - probably worth it. These modems above are pretty much supported by every cable Internet provider, but remember these units above do NOT have a phone jack on them for phone service. You can subscribe to a 3rd party VOIP (voice over IP) phone provider if you like.
If you use cable Internet WITH phone service, get a modem/router like one of these:
Arris Docsis 3.0 cable modem only with voice - NO WiFi: http://amzn.to/2ug1E8h
They have these used as well for less: http://amzn.to/2sO3FEd
Cisco Docsis 3.0 cable modem only with voice NO WiFi: http://amzn.to/2sNAmlm
Arris modem/router combo with phone HAS WiFi but the WiFi can probably be safely shut off (we have not tested this): http://amzn.to/2uc99Mu
Be sure to check with your cable provider to make sure that they fully support the model number of modem/voice unit that you are buying before you buy it. Make sure it is compatible with their service and they support it.
You may need a ROUTER to share the Internet connection between multiple devices. If you buy one of the modems above that is not also a router, then you should get this as well:
TP-Link Gigabit Wired-ONLY router: http://amzn.to/2sNJXJ2
Customer support service by UserEcho