It's hardly proof either way but some of my Raspberries with Node-red cannot connect to my 5GHz only router (192.168.1.1) so there is a 2.4GHz access point at 192.168.1.2. Nevertheless the Pies know the correct gateway:
route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
default 192.168.1.1 0.0.0.0 UG 600 0 0 wlan0
192.168.1.0 0.0.0.0 255.255.255.0 U 600 0 0 wlan0
Is it really true that a connection forwarded from beyond the router appears to have the gateway IP?
Using @marcus-j-davies' whitelisting strategy NR can easily whitelist all possible private IPs. But if the gateway IP has to be excluded it's much more difficult. NR might be installed at one location (school) to use at another (home) or used at multiple locations.
