You have to setup the Flint router as an exit node for tailscale. Tailscale will punch open the ports for you regardless of whatever your ISP is. Definitely should work on 4G GNAT Orange in Spain.
Then any tailscale device on the network will be able to use it to exit the internet. Performance should be the same as WG VPN (most of the time if you are not using tailscale proxy server). I frankly haven’t played with this myself ,but it’s totally doable. Either just touch the config files yourself on the router which will be tricky, or wait for gl-inet to add support for this. 4.2.x FW has tailscale but unsure what use cases do they support. The one I’m talking about is clearly the one they should support as this is the reason why virtually everyone is buying their routers (to peer traffic through your home inet)