Thanks MartinG: https://forums.frontier.co.uk/login also appears to work. Just remember, DON'T click in the login with Frontier box, just fill in the old name and password.Public service announcement: If, for whatever reason, you cannot or do not want to log in via Frontier Store, here is a link to the XenForo login page, where you can use your old forum credentials:
https://forums.frontier.co.uk/index.php?login/