As a long time lurker, I don’t have original info to add as I am based in the UK and only have my annual pilgrimage to go on. I am super thankful for all the info, advice and tongue in cheek banter on these forums.
We were also having trouble getting one day tickets, either because the sites recognise us as UK and offer different ticket options, the Universal app would not show beyond June (despite a whole lot of scrolling and refreshing), or the sites needed a US/Canadian credit card.
My wife found a tip on a facebook group however, switching iPhone settings to show Miami time meant that the Uni app now showed one day tickets through to our trip in September, 2 extra tickets each bagged . Thought this might help any others frazzled by the experience, and lurked no longer.