Connect extender/router to xfinity hotspot

Usually those xfinity hotspots are someone else's xfinity gateway. 

And usually extenders/repeaters still need a decent signal to connect to to rebroadcast. 

so if your phones are struggling to pick up the signal, its  doubtful the extender will do much better. 

You might have better luck finding out which neighbor has the xfinity hotspot and see if they'd be interested in sharing internet access at your cabin. You could connect them with a point to point system and simple pitch in a bit on the neighbors bill.