That’s the reason for these forums. You’ve picked the most complex issue where there is significant change. The summary is:
Apps like NextCloud do not have configurable IP addresses until Fangtooth and an App catalog refresh in June. 25.04.1 timeframe. Just keep using CORE until then if you wish.
You can run Portainer and Dockge as an App and then manually configure IP addresses for custom Apps, including NextCloud. That is in 24.10.2 and 25.04-RC.1
The Jail-like technology is called Jailmaker and is being replaced with Incus-LXC in Fangtooth. This allow IP addresses to be configured and then do a manual install of NextCloud. That is in 25.04-RC.1.
The 1st announcement of this is here: