I recently setup xfinity in my apartment and ive noticed i am getting very poor connection on everything wifi related. Phones are only getting 1 bar through wifi and anything that requires connecting through wifi cant get a strong enough signal to connect (Wifi Camera security system etc) Computers wont work on wifi either. The ethernet connection works fine. The router is up on a stand a few feet in the air within the same area as everything else trying to connect to it. Even having my phone within a foot of the router makes no difference in signal strength.
Although WIFI is great for not having to be tied to a cable, it's poor for signal strength. A signal travels far better through a cable than it does through the air. Any number of reasons it's weak. House wiring, appliances, ductwork plumbing etc can have a negative effect on wifi. You'll see the farther you get from your wifi router, the worse your speed will be. Another common mistake is placing the wifi router/modem in the basement and then expecting good results on the main floor. Another issue is older laptops with network cards that are only good for 54mbps. If you're going to use wifi, your best bet is to place the wifi modem/router to where you spend most of your time online. In your case, you might try changing the channel on your wifi that is less crowded.
FWIW, the poster stated this;
"Even having my phone within a foot of the router makes no difference in signal strength. "
You could try getting it swapped out for another and see. Even a different make / model number of the XB3;
Model Numbers: DPC3939, DPC3941T, and TG1682G
Good luck !