Occasionally, when a user inputs their credentials on the proxy portal and clicks Login, the page loops back onto the login page even if the credentials are correct.
This is a known browser cookie bug with a relatively simple resolution. The following screenshots are taken from the Chrome browser, but should translate to other browsers readily.
Navigate to the MobileHop login page. Right-click an empty space and click Inspect.
Near the top of the inspection window, look for the Application tab (may need to click the arrow to expand the menu).
In the left column, look under Storage and find Cookies. Click the little arrow to expand the Cookies, and you should see an entry beginning with https://portal.mobilehop... Right-click it and select Clear.
Then refresh the page and log in again. This time you should be met with terrific success.