5G inet plan switched without my authorization

I went into my local T Mobile store to exchange a defective 5G gateway that I only had for a few months, I was on the $30 a month 5g plan.  They said they would have to cancel the existing number of the old gateway and assign me a new number for the new gateway. I was told to call 611 to cancel the number.. All is fine until I had to call tech support for login issues with the new gateway and they said they could cancel the 1st number.  He tells me to log into my account to see what was done and I see that they CHARGED me $40 a month for the new line.   NEVER WAS i TOLD AT THE STORE THAT THEY WOULD CHANGE MY $30 A MONTH PLAN TO $40.  I NEVER AUTHORIZED THEM TO CHANGE MY PLAN.