I can't get my 4G Data to work on new phone

[AC Question]

I recently purchased an unlocked LG G4 Smartphone, apparently the previous carrier was AT&T. I have 4G Data coverage with my provider, T-Mobile, but upon putting the sim card into the new phone, this G4; I was able to make calls and use my house wifi, but when going out or the house the 4G doesn't function or turn on at all. I have the data enabled with my phone, what else can I do?
-Matt