Didn’t change a bit when it comes to these special redirect settings so it should work out of the box. The redirection of admin works like a charm, webmail not. Even if I use IP:20000 it does not work.
Perhaps your ISP blocks port 20000?
Some ISPs block “strange” ports, and if you’re behind some corporate firewall they may have some policies in place.
From my work laptop I can’t access either port 10000 or 20000. Heck, I can’t even access roundcube on on regular 443 because the URL has webmail in it