Here is a method I use that I haven't seen mentioned, I'm on a S5 SM-G900A at&t that is unlocked to T-Mobile, I am on Android 5.0 rooted stock. using the 80.00 unlimited prepaid plan. I assume this also works on the G900T:
I use FoxFi/PDAnet+ USB mode tether. I have the PDAnet windows program setting "Hide Tether Usage" Level 1 checked.
My hotspot data log from the T-Mobile app has not increased.
My current Data usage is at 13400mb/unlimited and my Mobile Hotspot has not increased ever since I did the PDAnet I mentioned above.
The other thing I have done which is also working perfectly is enable internet Sharing in network connections for the PDANET Broadband Adapter, which automatically changes my LAN adapter to 192.168.137.1... Then I have my LAN on PC plugged into a Router. The router's ip address is set to 192.168.137.254 and the DHCP server is turned off ( or forwarded to 192.168.137.1...probably if it is a Dd-wrt router) so now in my house, anything that connected to the router's ssid, LAN or wireless, gets blazing fast internet, all through the USB tether..
I'm telling you, not exaggerating, it smokes FIOS Verizon, I'm getting speed test results on all my devices connected to the router at like 30MBps-50 and above. All along I am monitoring the t-mobile "my mobile" app and seeing the Data increase and still the Hotspot total has not increased