Can’t get the 3yr Onstar plan that was included with vehicle

I purchased a used 2024 Hummer EV SUV used from a GMC dealer.  When I tried to activate the Onstar 3 year premium plan included with the car that is on the window sticker, they are saying since it is used it’s only 30 days. I know this is incorrect.  I have read several posts of other owners having the same problem but got it resolved.  I have tried Onstar many times and they keep escalating the claim and say they will call back but never do.  Who at OnStar can I talk to to get this resolved?