T-Mobile store (Mall of New Hampshire) Misrepresentation before purchase Manchester, New Hampshire

I walked into T-Mobile store with two AT&T phones and unlock codes.  The agent and I successfully unlocked both phones.  The agent promised that these two phones would be able to get 4G service on the T-Mobile network with a "Monthly 4G" plan from T-Mobile, so I purchased the plans for both phones.

Total cost: $60/mo. x 2 phones, + $10 activation fee x 2 phones, = $140.

After some hiccups, both phones were activated the following day, at the store.  I insisted on checking functionality of both phones at the store, before leaving.  Everything worked fine, except 4G service.  Only then did the agent (a different one) inform me, that these two phones would only get 2G service on the T-Mobile network, even though they were 4G phones, and were getting 4G service on the AT&T network.  Other customers had also brought their AT&T unlocked phones, and were getting 4G service at the same store, same location, same time, with no issues.

When I reset the phones back to the AT&T network, I went back to the store and asked for a refund.  No refund.

I contacted the T-Mobile Customer Relations department (they have no phone number available to the public, no email address, only a PO Box address in Albuquerque - Wonderful Customer Relations approach, isn't it?) and explained the problem in detail.  Their reply: No refund on prepaid monthly plans.

I found out later, that there is a T-Mobile web page that lists which AT&T phones may be compatible with the T-Mobile network after unlocking them, and neither of my phones was on that list.  Therefore, T-Mobile knew, was aware, that neither of these phones would be fully compatible, and failed to inform me of that fact.  If they had, I never would have bought their service.  This is not "buyer's remorse," this is mis-representation even if inadvertent.  The phones were active on the T-Mobile network for all of 3 hours, and they believe it should cost me $140 when it's their fault to begin with, for failing to provide me with complete information.