Tailscale is a heavy program, you most likely need to configure ex_root
to be able to install it.
https://openwrt.org/docs/guide-user/additional-software/extroot_configuration
VERSION=1.2.6 # Version as of Aug 11, 2025
wget -O luci-app-tailscale.ipk https://github.com/asvow/luci-app-tailscale/releases/download/v${VERSION}/luci-app-tailscale_${VERSION}_all.ipk
opkg update
opkg install ./luci-app-tailscale.ipk