Based on the tutorial*/ where is mentioned also server admin side of the setup.

But here i mention only how to setup it as a reseller and then as user (it appears that both needs to be done).

As a reseller once admin assign an IP to my account. I can see it in Dashboard / IP Management (https://DAhostnamehere:2222/reseller/ip-config) - there you can mark the ip as "shared" so you can assign it to multiple accounts

So i can assign it to the user by going to the list of users: https://DAhostnamehere:2222/reseller/users , clicking user, clicking Modify tab, Add additional IP

Then as user, i need to go to Domain management, click domain name, click Domain IPs, Add IP, selecting IPv6 and make sure DNS option is ticked, so it create appropriate AAAA DNS records. If DNS is managed on external server (like Cloudflare), you may need to update records manually on it.