I managed to enable the 'Exit Node' option on my GL.iNet router. It actually turned out to be very simple, just had to send the following command via SSH, and that was it. No additional tweaks, firewall rules, etc.
tailscale up --accept-routes --advertise-exit-node --advertise-routes=192.168.15.0/24,192.168.8.0/24 --accept-dns=false
192.168.15.0/24 - WAN Subnet
192.168.8.0/24 - LAN Subnet
Router details
Model: Brume2
Firmware: 4.7.0 (beta)
Tailscale version: 1.66.4.-1 (default on this firmware)
Rebooting or powering off the router doesn’t terminate the 'Exit Node' option.
However, if Tailscale is manually stopped in the web interface and started again, the 'Exit Node' option disappears, and the SSH command needs to be sent again.
If anyone knows how to automate this - sending the SSH command every time Tailscale is started - it would be very helpful.