When I tried this at the beginning of the year (internet service was down due to a major snowstorm, but our home LAN was working fine), it was unable to connect. Nothing had changed in our server / client config since it worked last, only our internet service had been cut. My understanding is that if the clients and server can't handshake through the SteamWorks API and Steam Servers, multiplayer can't work, even if you have clients and a server all on the same LAN that work just fine.