First you need to download the ipk
wget https://downloads.openwrt.org/snapshots/packages/mips_24kc/packages/tailscale_1.8.1-1_mips_24kc.ipk
opkg update
opkg install tailscale_1.8.1-1_mips_24kc.ipk
Installing tailscale (1.8.1-1) to root...
Collected errors:
* satisfy_dependencies_for: Cannot satisfy the following dependencies for tailscale:
* tailscaled
* opkg_install_cmd: Cannot install package tailscale.
But still lack of one package which you need to download and install at the same time.