I created a short tutorial on using sub domains to access services hosted within my home network, thought I would share it here in case anyone finds it useful
This is the first time I’ve made a technical tutorial so apologise if there are mistakes/its confusing, feedback will be appreciated
You seem to have descibed your port forwards backwards It is the router forwarding the ports to the gateway pi (and potentially other devices), not gateway pi and other devices forwarding to the router. The forwards to servers are incoming from the internet.
(Theoretically you could have your pi physically between the router and the internet (modem) acting as a sort of pre-router, but this would be unusual. Perhaps you could describe your physical setup more clearly. What is physically/wirelessly connected to what, to the internet.)
He does refer to the pi as a gateway, so you would be right about it coming before the router. In that case, the pi would be the device handling NAT and forwarding ports.
So I think he’s describing it accurately… it’s just not a common setup to see these days.