Confirm that your tablet is connected to the same wireless network as your Square Register. Make sure there are no firewall rules blocking the connection of the devices. Try turning off your tablet and reconnecting it to the network. Make sure your wireless router has UDP Broadcasting enabled.
Firewall rules: tcp port 9100 and 9101; udp port 22222.
Power off your Wifi router and tablets. Power on your Wifi router then your tablets.