Disabling Internet slows down Home network to almost nothing

Hi there
Is there something in modern Routers that block network when Internet is disconnected.

I'm trying to run a simple Home server to serve movies / music from a NAS server without Internet connection (deliberate) but with Wifi.

When I disable the Internet system shows Limited connectivity and I can't access any computers on the LAN. Once Internet is enabled again it all works perfectly.

Is there some setting on the router so I could operate the LAN as an "Internal LAN" isolated from the network.

If I set the NAS up as a gateway it works fine - but once I remove the Internet connection it goes bonkers.

Any settings -- commercial routers (expensive) don't seem to have this problem - especially if you can add switches.

I just want to do this with a bog standard home router.

Cheers
jimbo

Is your router set up as the DHCP server?