Hello Peephouzoaz35
Yes, you need a ticket from the last stop of the train in a Mobilis zone to the first stop in an Unireso zone. If you take a RE train, it is between Coppet and Genève. If you take a RL train, it is between Mies and Pont-Céard.
You should buy a ticket with the Upgrade City Genève, I think that it is cheaper as a separate Unireso Day Pass. Example Coppet - Genève Half-Fare Return:


Kind Regards
Danhiel