You’ll have to do some things on the command line.
sqlite3 ~/.mozilla-iot/config/db.sqlite3 "DELETE FROM settings WHERE key = 'notunnel'"
sudo systemctl restart mozilla-iot-gateway
After that, the gateway should prompt you to set up your domain again. To reclaim an old one, you’ll just need to put in the same subdomain and email address you previously used. It’ll prompt you to reclaim, and then you’ll get an email with a token, which you’ll need to put in the form.