My case:

  • Router: GL-AXT1800
  • OpenWRT version: OpenWrt 21.02-SNAPSHOT r16399+159-c67509efd7
  • Kernel Version: 4.4.60

Tailscale installed, added tailscale0 interface, firewall and NAT rules in LUCI
I have the same issue that has @lextar but I solved using this CLI command

tailscale set --advertise-routes=xx.xx.xx.xx/24
tailscale set --accept-routes
tailscale up

Then I can reach any other tailscale device.
But, If I reboot the router I loss the config and I need to make again the sames changes.