Unless there’s some kind of restriction on your T-mobile account:
Simply shut down your current phone, remove the SIM card, make sure the Librem 5 is shut down, insert the SIM card into the L5, and start the L5. APN should set automatically.
You’ll need to enable VoLTE, which setting can be found in the BM818 Tool.
(Your phone number is associated with the SIM card, not with your original phone… unless it’s an e-SIM.)