Hey OP here - I never got it to work. I tried everything from trying to select the property first, to setting up an account first, and whatever else. It just wasn’t having it! Just left Russia last night after 18 days - I booked all of my private flats with Ostrovok and everything worked out fine.
If anybody is from the UK here, I can say with that Santander and Monzo work with that site well (as it winds up being charged in BYN).
I’m in Belarus now - not a single one of my cards has worked in ATMs or Belarusian websites. Except for Monzo (and of course my Mir card).