Can't choose "custom eth0" as network type for Docker

Unsure if this is a Bug or if I'm the "problem"

I had till date a "Big" System which was only started "on demand" wiich holds the storage (#1), and another system (Tiny - #2) for 24/7 Services like Immich, Vaultwarden etc.

But somehow i thought this 2 systems (which are also a littled aged) shouldn't be necessary, so i bought a new Ugreen NAS DXP2800 which should become my 24/7 all purpose Server.

Everything works on this Ugreen, though i can't fully transfer my Dockers as some of them are using their own IP in eth0 Network.

But when i try to configure this now on the Ugreen (using the USB stick from #1) it doesn't show this option:

I think my Docker- and Network config should be ok:

So is there a Bug? Or where is my mistake?

unraid-diagnostics-20250406-1117.zip