This readme should help you with tailscale client setup.
There are two packages related to tailscale. Tailscaled (daemon which has to run every time you want to be connected to VPN) and tailscale (package with a utility which is necessary for registering device).
To install them run
opkg install tailscale tailscaled
Note
By default this package will use nftables. If you wish to use iptables, the config file /etc/config/tailscale can be modfied, changing the line fw_mode 'nftables' to fw_mode 'iptables'. You can then run /etc/init.d/tailscale restart to restart tailscale using your chosen method
First, enable and run daemon
/etc/init.d/tailscale enable
/etc/init.d/tailscale start
Then you should use tailscale utility to get a login link for your device.
Run command and finish device registration with the given URL.
tailscale up
See the OpenWrt wiki for more detailed setup instructions