Thanks @snook - but I can't even get the page to appear on the big computer, never mind logging in, and when I tried to upload I just got a whole series of error messages. I worked on it into the small hours over several evenings, posted support queries on their Discord – some answered, but to no avail.
Also I noticed that there were lots and lots of support queries similar to mine, so I think maybe the technology I'm using isn't supported – even the tablet computer I bought specially for using Dtube! But I've tried a couple of other video platforms and had no problems uploading.