Yes - that was my thinking - but I'm not able to climb up on the roof.
I would have thought that a local aerial installer would be prepared to go up there and change a SIM for a small charge.
Indeed. I would have thought that installation of "rooftop" mobile broadband devices would be a nice sideline for any sort of reputable aerial installer. There is minimal additional equipment to buy (cable tester, crimp and punch down tools) and they are good to go.
Obviously there are the cowboys would won't know what a spectrum analyser was if it bit them in the backside (and are hence not reputable IMHO), but aerial/satellite installers are in general ideally placed for this sort of work.