T-Mobile's $30 unlimited data plan?

After your 5GB you will only receive 2G data until the cycle renews.

The point of this is 5GB can be considered excessive. So after 5GB you get slower data causing you to use less.

T-Mobile reference guide:
LTE = 4G-LTE, real 4G
4G (HSPA+) = "fake" 4G, it's really 3.5G which some people call turbo 3G. they call it 4G for marketing purposes
3G = 3G
2G = "E" or "EDGE" data

Yes, after 5 GB used of 4G speeds, you'll be downgraded to 2G speeds for the rest of the billing period. Then it resets.