I'm still looking into the correct command to execute for my case, but still no clue. (Maybe I should consult a specialist...?)
Iptable looks a bit too advanced for beginners. Still, limiting access to my.yourcloudron.com by IP is one of the essential security features, I think.
If Cloudron UI can let us manage IP address whitelist/blacklist, that'd be a great help for me and people like me.