Bought used NOK5G21 - TMO says it's blacklisted ? Can I activate as new customer ?

If by some chance you bought the gateway on eBay, you may be able to get them to help you, as you essentially were sold stolen goods. As JMA mentioned, T-Mobile doesn’t sell them. If you “lose” one or damage it (for example, by trying to modify it), or if you cancel service without returning it, you would legally have to pay T-Mobile for it. It sounds like maybe the seller cancelled service (or just stopped paying), and then sold the gateway instead of returning it.  If you bought it locally, say through Craig’s List, you could contact the police and report it.  As bsc mentioned, you could just try to order the service as a new customer, in which case T-Mobile will send you a gateway without charge, as they did for all of us. Thre’s no rental charge for the unit, so you’d be paying the same for the service whether you had your own gateway or not.  You’d still be out what you paid for the gateway, but at least you’d have the service while you try to get your money back from the seller.