We haven't asked how this isolator is wired.
I, at least, have assumed it is a three-pole isolator but does it isolate the light as well.
How is the fan controlled? By this switch or by the light?
Why not do away with the "isolation" part and just swap it for an ordinary switch that controls the fan - in one way or another?
The fan and its light go through a three-pole isolator. the fan and its light switch on when the isolator is on and then the light switch is put on, this switches all bathroom lights and the fan on. when the lights are switched off the fan is then on a timer.