Grandfathered Simple Choice

I am also on a grandfathered plan for over a decade. And recently around 3rd week of July this year we decide to try a Mobile hot spot (kid homework on the go) with 10GB plan and the representative told me I don't have to worry about which plan I need yet for now just pay for taxes and fee to start my 30 days free trial. She promised to call me back before 30 day period ended (should be 1st week of Aug) but I've never received any call back at all but when I talked to the agent last week, they kept saying that someone was calling me and left the voicemail even though I've see no any voicemail on the day they're claiming. And the agent told me that I can't return the device anymore bc it's over 20days old (*no one ever told me about the 20days policy at all in the first place) from the day I sign up for the trial, I was trying to explain the situation still no use, they said I have to keep paying for the device bc they can't accept it back even though I don't use the service that it didn't work!
I was told by one of the representative that the company doesn't offer much to us with grandfathered plan bc we're not new sources of profit to them so there's not much room to move at all either switch plan or closing the account, they're targeting new customers and/or higher rate plan customers only with many promotions and stuffs. Very disappointed… considering different carriers now :((