Ok I put my original SIM into the new iPhone. Text Messaging works, but data doesn't. The notification bar says T-Mobile 4G, but safari and chrome can't access the internet. iMessage also wont send. So data isnt working for sure.
Call customer support. The same thing happened with mine. They will need the number for the SIM card.
You need to check the APN setup in your new phone. If your new phone works (including WiFi Calling) with the old SIM then you should be able to keep the new SIM as a spare. If not, you can do a SIM swap online using My T-Mobile.
Go to your settings> general > reset > reset network settings..phone will restart and it will wrk.
Though you also have to check the settings> cellular > make sure cellular data and data roaming are both on and that the apn is set up to epc.tmobile.com.
Retrieving data ...