Run am Ethernet cable. If there is no way to do it inside the house, use some sort of conduit to run it outside and/or cable intended for outdoors use.
If you already have a spare wireless router, you can use it as a wireless switch, but you'll be limited to 3 devices (if you want your whole house to be on the same LAN).
Recommended way would be to get a switch and an access point (which would be wired to that switch) for your second flour. Make sure DHCP server is disabled in access point. You can setup both wireless networks completely the same (except for probably the channel), or you could distinguish them (like Flour1, Flour2) to make sure you always connected to the most appropriate WiFi network.
If you already have a spare wireless router, you can use it as a wireless switch, but you'll be limited to 3 devices (if you want your whole house to be on the same LAN).
Recommended way would be to get a switch and an access point (which would be wired to that switch) for your second flour. Make sure DHCP server is disabled in access point. You can setup both wireless networks completely the same (except for probably the channel), or you could distinguish them (like Flour1, Flour2) to make sure you always connected to the most appropriate WiFi network.