[Request]

Im no expert in Wifi hacking, but if the network is WPA or WPA2 you can try to capture the handshake and crack it with Aircrack. However this method is only good as your wordlist. You could try to crack the WPS pin with reaver though.

Note that capturing the handshake requires a device connected to the network that you can intercept the connection from. In the case of the billboard it might not have any devices connected to the wifi, and instead the control systems most likely run over ethernet.