A extender/repeater will damage you primary signal. It transmits a copy of the data back into the exact same channel causing interference.
Pretty you trade coverage for speed/quality. The trade off is when a poor signal is better than no signal.
The other nasty issue is some routers....mainly mobile broadband hotspots....do not support the WDS protocol that is required to make a repeater work. I would bet the provider router does support WDS, it likely is a option in the settings that can be turned off and on.