Dehumidifier cannot cope with moisture produced in a short time , opening window cools room rapidly encouraging heavy condensation .
Bathroom needs to be warm so it supports airborne moisture long enough for fan extraction ( which you don’t have?).
Or just don’t pay tv licence , my brother in law and my brother have never had tv licence and never been fined , they just shut the door on anyone making enquires .
Should be easy fix , carefully cut off rusted screw and remove upper mount , if you can get same rail for replacement mount 10mm lower so you get new screw position and old screw position is hidden by the post base .
If needed you can increase post bases dimensions with a large chrome washer...