As per the title, with clients who both have DHT enabled, surely this is a situation where this would work? I have done the same with other torrent clients, Tixati is the only one where I have had a problem.

Works fine for me. What do you mean by unconnectable? You need to know if the other client gets your IP from DHT and is unable to connect, or doesn't get your IP at all.

Sorry, they don't get my IP at all.

See in Settings if "DHT mode for trackerless transfers" is set to Search + Announce. Even if Tixati doesn't announce, it should find peers after DHT search, that is every half an hour. Most clients don't show all peers' addresses. They show only connected peers by default. Maybe they get your IP but can't connect due to firewall or NAT. In that case you should follow the Optimze Tixati instructions.

BTW I noticed Tixati doesn't log DHT announcements anywhere. It logs only searches, I think it should log announcements too.