You want the public IP address to be presented to the
ER-X--not to any device before it.
It's a simple test: run the wizard to configure ER-X for
WAN-to-LAN; configure the WAN as dynamic IP address.
When you plug the Internet service into port0, the ER-X
dashboard should show the WAN IP address in service.
If that address is a public IP, it's good. If it's not--for example,
a 10.10.n.n or 192.168.n.n address--it's double-NAT. Dave