You might need to change your APN settings:
APN: epc.tmobile.com or fast.t-mobile.com (for LTE devices)
Proxy: <Not set>
Port: <Not set>
Username: <Not set>
Password: <Not set>
Server: <Not set>
MMS proxy: <Not set>
MMS port: <Not set>
MMS protocol: WAP 2.0
Authentication type:<Not set>
APN type: <Not Set> OR Internet+MMS (depending on software version)
According to T-Mobile, the current firmware for this model is Android version 4.3. Flashing Lollipop is unsupported, and the firmware that you flashed has not been tested by T-Mobile. Thus, you're pretty much on your own from here, but perhaps someone else in this forum will have other suggestions.
You flashed a new baseband. Flash back to your original firmware and then check XDA on how to flash the firmware without flashing the baseband.