@dedecam 

If you use the X50 in access point mode you should be able to extend the signal of your bell router but it won't be a mesh network. You will need to connect the deco with an Ethernet cable to the bell router first and set it up then you can switch it over to access point mode. This link goes over how to set up the deco for the first time 

This link goes over how to switch it to access point mode after it's set up

As for the range it can cover up to 2,500 sq. ft. per unit