It seems as though the dimension does not appear in the mod's dimension wrapping feature. Seeing as this is an underground cavern mod, I think it would be very cool to have the option to place the dimension under the overworld.

What versions are you playing on? I am on mc 1.15.2, forge-31.2.36, and undergarden 0.1.5 and doing the dimension stack feature without a problem.

I'm having the same issue as OP, and I'm on:

Minecraft: 1.16.1
Forge: 32.0.108
Immersive Portals: 0.2 (Even though in the in-game mods list it says 0.1?)
Undergarden: 0.2.3

immersive portals isn't supported for the 1.16.1 version of the mod, however the 1.15 version is

It's unfortunate that things must always be this way. Oh well, these things happen when you've got an infinitely-progressing game like Minecraft. As the old saying goes, progress waits for no man.

he'll probably work on compatibility once he comes out with the 1.16.2 port lol

mod compatibility isnt an issue.