Good question billy626 and I have a few thoughts on this. Are you sure this phone can connect to the band frequency on our T-Mobile network bands & technologies page? There really aren't any settings other than the normal APNs we enter for android phones. If the phone supports the frequency, it should connect. I do have to warn you that we can't guarantee it will since it's a Non T-Mobile phone.
According to Samsung Galaxy S6 (CDMA) - Full phone specifications, it does, in fact, have the bands required for T-Mobile.
You said you have a G4 with another line that doesn't appear to have issues. Have you tried that SIM in your S6 to see what networks pick up? If it picks up LTE, then it's something with your account that would have to be looked into. If it doesn't pick up LTE, then I'd wager there's something with the phone and how it needs to be configured. With that, you'll probably have to contact the experts in that device over in the Sprint forums or maybe xda.
On the same model Verizon/Sprint phone, with factory reset and inserting my existing T-Mobile SIM card, I am having success getting LTE network access using Automatic as the Network Mode. Details below - I think I set up the S6 differently after the factory reset.
I did a factory reset on a Verizon/Sprint Galaxy S6 Model SM-G920-P.
I had to log into Google to start with due to factory reset protection.
I migrated all available data and setting using Smart Switch over wireless from old phone to new phone.
I moved my T-Mobile SIM card from my HTC U11 phone to the Galaxy S6.
I didn't have to enter a serial number or IMEI number for any setting.
I got a text message giving me a TMPProfile setting for Access Point Names.
In Settings | Connections | Mobile Networks | Network Mode, the choices were CDMA, LTE/CDMA, GSM/UMTS, and Automatic.
I selected GSM/UMTS and phone would not make/receive calls, then I set Network Mode to Automatic, and for two days I show LTE network and the phone portion works, as well as texting, Wi-Fi.