I have never once successfully gotten a laptop to work on a united flight. Heck I recall using it back in 2016 (might have been 2017) and it wouldn't let me due even though I still had flash setup on the computer. The webbased software just flatout doesn't work and in my limited experience has never actually worked on computers.
(I tried IE, Chrome, Chrome with every extension disabled, Firefox, and edge)
I would suggest saving some videos for offline playback instead. Now this is honestly a travesty as United does have a pretty decent library of shows and movies with some real winners in it.