Not familiar with the Bell units, but I would disable the WIFI on it if possible and only use the Ubiquiti APs. Have never had problems with Ubiquiti APs in houses and have them on many different providers (we don't have Bell Fibe in our area). Also make sure the APs are set properly.

Lastly if they are using iPhones you may want to reduce the broadcast strength on the APs. it's a known issue with iPhones in that they will hold onto a connection for as long as possible. If they were originally connected to the Bell unit and move to under the UBNT AP but the Bell signal is still available the phone will stay on the Bell signal. By disabling the Bell WIFI and reducing the broadcast strength you are reducing the overlap and forcing the phones to rescan for the stronger signal. This may also require you to add a couple more AP's to help fill in dead zones