Your modem may offer two different frequencies for your Wi-Fi signal.
The 5 GHz frequency offers greater speeds than the 2.4 GHz frequency but offers less range and has a harder time dealing with obstacles. The frequency your device should use depends on where in your home you are using it.
Bell modems will attempt to select the best frequency, however, devices ultimately decide the frequency to which they connect. Sometimes devices connect to a frequency that is not suited for where they are being used in your home.