MultiWAN: Portforwarding works, NAT not

[m4rtin]

Hallo, I have a 2 WANs configured with the HowTo of the opnsense website. I can access the Opnsense from the internet via portforwarding.
If I try to access for example 2 of my synology diskstation (port 5001 and 5101), then I see the incoming request coming from my public IP to the correct IP and port of the diskstation. But I get 404 error in the browser:





This is my Gateway Group:



This is the NAT rule:



The 2 NAS IPs can be pinged by the internal ping tool of the opnsense via LAN interface.
Do you know why it doesn't work?