Disable internet connection but leave the LAN working on windows 10

You can create local area connection without having internet connectivity. If the network is already set up, then what you have to do is to just to just plug off the broadband/DSL modem with the router, the internet connection will stop working, but the LAN will continue to work.

Or to setup LAN without internet, first connect a switch with the router LAN to LAN port, then run ethernet cables between the devices. Then run Network setup wizard.

Or to setup internet-less Wi-Fi Make sure all devices have Wireless network adapter installed, and most wireless routers are capable of working as an access point so you don't need a switch. And then you have to configure DHCP on router, so connect a wired device with the 4 built ports on router. Access the router's setup page (usually http://192.168.0.1) from that device's browser. Login as administrator. Set the DHCP address pool, and DNS server is not required as you are not connecting to the internet. Go to the wireless section and set the SSID of Wi-Fi, Pre-shared key, enable WPA2-PSK or WPA-PSK authentication if supported and set a strong password. Now connect all other devices to the Wi-Fi.