USG20-VPN NAT does not work after router substitution

I am having difficulty activating a simple NAT on USG20 (firmware 4.62), after a VDSL router replacement.

The router have a public internet IP (5.x.y.81), and a LAN IP (192.168.0.251/24).
I cannot operate on the Vodafone router and, therefore, I must "trust" that a 1:1 NAT has been configured between all its ports to the USG (as requested).

The USG WAN port is 192.168.0.9/24 (default gateway of course 192.168.0.251), and the is LAN 10.0.0.248/24

On the LAN (via a switch) there are various servers, for my tests a NAS, with IP 10.0.0.119/24, gateway ANOTHER USG (10.0.0.254), various service (for test say 23 telnet)


I want a NAT from the router to the NAS, telnet 5.x.y.81 (from outside) should go to => 192.168.0.119:23

It sounds simple, but I doesen't work.

For testing purposes I can even keep the firewall (policy control) disabled.

It is possible to reach the https interface from Internet (https:5.x.y.81), and even FTP, SSH etc (ssh 5.x.y.81) of the USG (test configuration of course).

By verifying from the web console (of USG) it is possible to telnet 10.0.0.119 and log in, therefore the USG <=> NAS traffic should work

In theory, a NAT rule should  be enough (and then the firewall rule but, as mentioned, for testing I can also leave it disabled)



I can reach the Zywall (even FTP, just for test, https etc), from Internet


USG can "speak" with the NAS, as it shoud be (telnet for test)


BUT, when I try telnet 5.x.y.81 (or whatever, ssh, https etc) => impossible.

By setting a specific policy (to be able to log) everything seems to actually arrive as expected, in this case from my external internet IP (nas1 = 10.0.0.119)
The same problem occurs for services other than telnet (ssh etc).

Thanks for any helps