How to config for a network mapping (Arris/Starlink conflict)

[dolfs]

Scenario: I have two devices (Starlink and Arris SB8200) that both insist on using 192.168.100.1 for their management interface.
I am connected to the Starlink via Tailscale and a subnet router (it is in another location). Neither Starlink, nor Arris allow for their management network IP to be changed!

The connection to the Arris is on the WAN port (which for normal operations uses DHCP to get an address with Comcast). I currently have a static route for 192.168.100.0/24 to the WAN interface. This works to access the Arris.

However, I cannot access the Starlink this way because of the conflict. What I want to do is to make it appear that the Arris uses 192.168.101.0/24 instead by configuring some kind of NAT (or whatever) so that if from the normal LAN side, I type 192.168.101.1 it goes out the WAN interface to 192.168.100.1 and responses come back to the LAN appearing to come from 192.168.101.1.

I am sensing I need some combination of Virtual IP (Alias and/or proxy) and a 1:1 NAT mapping.

I'll take any advice or suggestions.

[Maurice]

This should be doable by combining port forwarding and policy-based routing.

Port forward rule: Interface LAN, destination 192.168.101.1, redirect target IP 192.168.100.1, set local tag 'Arris', filter rule association 'none'.
Firewall rule: Interface LAN, gateway WAN_DHCP, match local tag 'Arris'.

The first rule changes the destination IP address to 192.168.100.1 and tags the packets. The second rule forces the tagged packets out of the WAN interface.

Cheers
Maurice