I'm pretty sure it's explained in the wiki article on tailscale, but I haven't managed to do it.
To detail my problem, I have two routers, running the latest version of OpenWrt. The home router has Tailscale set up as an exit node and I want to set up the travel router to use it, but I don't know how. I've tried using tailscale up --exit-node=MY-EXIT-NODE --exit-node-allow-lan-access=true, as per this section of the wiki, but it didn't seem to work, so I'm not sure what I'm missing. Maybe I need it to advertise the subnet routes advertised by the home router?