Ha! Inspired by this thread and a previous awful experience with TalkTalk and another remote access program, and the fact that the RU server wouldn't ping, I set my DNS server to be Google's (others are available, of course, and I'll possibly change it to Cloudflare's ), and instantly it worked.
Thanks for nothing, TalkTalk.
Again.
Hope that helps someone. (Still can't see how to start a new topic: sorry. Maybe need to see SpecSavers.)