OK, for many IT people this will be obvious, but it blew my mind just now.
I have NextCloud running in an LXC container in my NAS, and I was looking for an easy way to access it over the LAN.
I usually access it over Tailscale but it feels silly to access a local service over a VPN.
And thanks to this video I learnt that I can add a route in my Mikrotik router config to just route the LXC network through my NAS as a gateway. And it works!