I just did this. I paid off my Sprint Note 5 (N920P03) and put my T-Mobile SIM in it. I called T-Mo and got these instructions to get it working. Only gets up to 3G, but on fast.com I get 12 Mbps down in my area, which is acceptable considering it wasn’t designed for T-Mo’s network. It’s good enough for YouTube, Netflix, etc. Plus you can still use Wi-Fi.
System Settings -> Connections -> Mobile Networks -> Network Mode (select GSM/UTMS).
Below Network Mode go to Access Point Names and “Add” (top right corner). Enter these details:
Name: T-Mobile US LTE
Proxy: not set
Port: not set
Username: not set
Password: not set
Server: not set
Multimedia message proxy: not set
Multimedia message port: not set
Authentication type: not set
APN type: default,supl,mms
APN protocol: IPv4/IPv6
APN roaming protocol: IPv4
Mobile virtal network operator type: none