I remember a previous poster said there was a way to fool the wifi system into thinking your browser is a smartphone. Here it is. I am on a 737 this week and will try:

"In Chrome, you can actually just open an incognito window > right click anywhere > Inspect > click on the second button from the left on the top left corner (the one with a phone and tablet) > select any phone (I pick "iphone 12 pro") > navigate to unitedwifi.com and you should see the T-Mobile option > after you activate it, you can unselect the device button and/or return to a "normal" window"

You can open an incognito window via File - New Incognito Window. Then Inspect opens on the right of the current page. Then the little phone/tablet icon is very small in that window, top left - 2nd button. Then in the left portion of the screen you can hopefully see a pull down of screen emulations. Then I assume you can just type "unitedwifi.com" into the Url bar and goes to the page. If you need to emulate different phones/tablets, you can click the settings gear icon in the right side. Looks like Chrome has some standard ones like iPhones already selected. Hope this helps. Will let people know if this works.