It is very hard to find the detailed information about the Internet plans on the T-Mobile website.
I found the answer here:
2GB Data Plan
Up to 2GB of 4G LTE data Internet Only
This T-Mobile plan offers max speed of 12.3Mbps in selected areas.
This is the price for 2GB of stand alone Internet.
After the first 2GB are used data speeds are slowed for the remainder of the billing period.
After a while I found this on the T-Mobile Support web site:
Example: You have the Unlimited 2 GB data feature that provides 2 GB of full-speed access on the T-Mobile network. You use the data feature for web browsing at full speed, and to download videos, pictures, and email. Once you use 2 GB of data, your data speed is reduced for the remainder of the bill cycle, but you still have unlimited access to data on the T-Mobile network.