I don't know a lot... but sometimes if a sink, bath or toilet hasn't been used in a long time/isn't used regularly there's no barrier of water in the S-bend to stop sewer smells coming back up. If it is due to that it can help to use it occasionally to fill the S-bend back up.
Only mentioned that on seeing the words 'guest toilet' and hoping that was one quick fix to reduce the number of problems, but IANAP (I am not a plumber).