New AT&T unlocked g flex 2 on tmobile. Data not functioning!

I jumped on the 0 eBay deal for this phone. Today, after installing my tmobile SIM card I can talk and text fine but the mobile data is not functioning. I've gone into the "Data Manager" menu and turned on the "mobile data"...airplane function is off....performed a factory reboot, and still nothing. What am I missing? Do I need to do an APN configuration? Being an unlocked phone, I thought it was a matter of plug and play. Data works fine in my moto g. Please help. Thanks