Could you clarify your network topology and the actual requirement?
From your description, it sounds like there is a primary router and a secondary router in your network, each with its own subnet, and you would like to run Tailscale only on the primary router while advertising and allowing access to both subnets.
If that is the case, we will likely need to configure it via SSH. The process is relatively simple and should remain effective unless the router is reset or the firmware is upgraded.
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On the primary router, follow the guide below to advertise the secondary router's subnet in addition to the primary router's own subnet:
How to advertise the router's WAN parent subnet/route to Tailscale?
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On the primary router, go to LuCI → Network → Static Routes and add a route to the secondary router's subnet.
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After that, everything should work as expected, and remote Tailscale devices should be able to access both subnets behind the two routers.
