another option is a homeplug wireless extender where you plug the first into the router and an electrical outlet and the repeater into another electrical outlet where you need it extended. With this being a guesthouse if they come off the same power meter as the house it may work, if they have their power meter this may not. With your situation it may be a try and see.
Yes, I agree with above the best would be to run an ethernet but in case that isn't an option this might be an alternative