If you're blacklisted there's not much you can do. You have to call at&t and have them remove the block.
Verify you have the correct APN settings.
APN:*epc.tmobile.com* but you itís an LTE Device, use: fast.tmobile.com instead
Proxy: <Leave in Blank>
Port:*<Leave in Blank>
Username:*<Leave in Blank>
Password:*<Leave in Blank>
Server:*<Leave in Blank>
MMS proxy:*<Leave in Blank>
MMS port:*<Leave in Blank>
Authentication type:*<Leave in Blank>
APN type: default,supl,mms
APN Protocol: Leave it to the Default one
Who unlocked your phone for you? Changing imei numbers is wrong and possibly illegal depending on where your at. Phones can be unlocked with out changing the imei. If someone changed your imei number to unlock it, I would contact them again.