T-Mobile will block you if you tether on Monthly4G. Your phone data will still work, but if you use it for more than a couple minutes, the tethering will start going to dead links.
T-Mobile's tethering block works like this: They sniff your unencrypted HTTP traffic for user agents that look like they came from a Windows machine, and if they find one they hijack the connection and redirect you to their tethering upsell page. They can't sniff HTTPS connections, so HTTPS continues to work. For HTTP you can change your user agent, or just use a Mac (which their sniffer isn't smart enough to detect, yet).
If their sniffer becomes smarter in the future you can always switch to a VPN for your tethering traffic, which they wouldn't be able to distinguish from a VPN connection made by the phone itself.
Ultimately, as long as you have full control over your phone they can't stop you from doing what you like with the data you're paying for. Of course, I wouldn't recommend relying on routine tethering without paying your carrier for it, but it's really nice to have on occasion.
This has not been my experience. I have never been sent to dead links.
I have had them try to send me to a "mobile hotspot feature" upsell page when using Chrome from my laptop while tethering. I figured out that they were checking browser user-agents to determine whether a user was tethering or not.
But the Chrome app on Android allows changing the user-agent to the desktop equivalent with its "request desktop site" feature. So I called and yelled at them saying that they had no right to block me from using my phone in the way that I wanted. They flipped a bit somewhere and I haven't had a problem since.
