The majority of your use needs to be on the Tmobile network.
gramps28 is right. The majority of the usage will need to be on T-Mobile's network. There isn't a specific limit T-Mobile can give you. It's all based on how much usage you have on T-Mobile's network vs whatever roaming partner you're using it on. You'll get notifications if you're the primary about your service when the roaming usage gets your domestic usage. You'll want to Contact Us and get in touch with Customer Care to go over your options since you'll be gone so long.
As for the 4GLTE, you'll be able to use that service if you're connected to a supporting roaming partner and have a capable device. You can use our International Plans | Traveling Abroad without Roaming Fees | T-Mobile page as it will give you a way to check our coverage map and check the device compatibility.