I'm trying to get my Ardor wallet working after several years of inactivity. I've updated Java and the 2.3.4 client. I can get to the wallet and it recognises my pass phrase and shows the old balance. However, it also says "Blockchain download halted, no connection", and the Peers page shows no connected peers. If I try to add one manually, it appears for a few moments and then disappears. I have port 27874 open, according to a couple of other websites I checked with.
What do I need to do to get the blockchain downloading? I've spent some time looking for FAQs, but I've not found anything that covers this.