If you think about it ... the mains plug only switches the live, the neutral is unswitched. you can happily feed through the relay. Solid State relays are prone to not switching off though. I use a 4PDT relay with all 4 poles switching the current. I was concerned with the inductive load of the router causing contact burn with just one pole. my 1020W motor is capable of drawing over 4 amps on load, I lacked confidence that my relay contacts were up to that :-)