SO no rotom for me or is it random?
I think what you have to do is catch the Rotom in the Old Chateau, then go to the third floor of the Jubilife TV station and talk to the TV producer. When he asks your opinion on TV select "Everyone Happy" and when he asks Feedback on TV select "Wi-Fi Connection". After that, save and restart the DS. If Mystery Gift appears on the Platinum start up screen it should have worked.