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.