Mtn locked sim 2 on prepaid

I bought a Redmi 9A on prepaid it's a dual sim,for my mom from pep cell, the cashier told me that sim 1 slot will only use mtn then the other will use any network. When we get home all slots wants mtn, slot 2 doesn't work even if we inserted mtn sim on slot 1.

Went back to the shop they told us there is nothing they can do because mtn is the one that blocks slots. I am trying to understand why would mtn on the phones package state that slot 2 will work with any network as long as slot 1 is mtn?

This has really decreased my love for mtn.