Why is nothing spawning in my Minecraft 1.16 Zombie Piglin farm?

I rented a server through Nitrado a few days ago with 3 other friends and now after a while I have built an XP farm. To be precise this

But now I have the problem that not a single mob spawns up there.

-Difficulty is set to hard

-I have already set "Hellworld = true" in the server settings

-Server has been restarted several times

Have you made it at the right height for sure? (240 or so, I think he says in the video)

Do mobs even spawn with you in the Nether?

It's built above height 230 and generally all mobs spawn as usual