I’ve had this problem as well for the last week or longer - it was very frustrating, seems to happen on any mobile browser. To replicate you need to be logged out, then navigate to a t-nation thread that requires login to access, and the banner will block you from logging in. The key is that you need to be loading a forum post URL to trigger the banner - if you just navigate to t-nation.com you don’t see the banner.
This is a problem on PC as well, for example, try and load the following URL on PC in an incognito window that is not signed in. You’ll see the banner. Try and press “log in” and you won’t be able to see the login form - at least on Firefox for PC: