A bad redeem code of xbox game pass ultimate

I bought a 3 months subscription for game pass ultimate and when I try to redeem it, it shows that it`s only for one month. I`ve already talk with the store where i bougth it but they said that it was a error from the code itself so I think I need to reach
someone from technical support but i don`t know where to put my complain, if you know something that I could do I will appreciate it.

Pd: sorry for my english, it`s not my first language

:)