This means the servers in your network cannot reach APNs. To talk to APNs, port 2195 for gateway.push.apple.com should be accessible from your server. To test your network, you can use netcat, telnet or openssl client. For example:
1) Use netcat to test the connection:
nc -zv gateway.push.apple.com 2195
It should report something like "Connection to gateway.push.apple.com port 2195 [tcp/*] succeeded!”
2) Use telnet to test the connection:
telnet gateway.push.apple.com 2195
It should report something like "Trying 22.214.171.124… Connected to gateway.push-apple.com.akadns.net"
It is probably the firewall in your network prevents your servers from talking to APNs gateway and makes the connection invalid. If you can confirm this is a network problem, you can configure your firewall to make the connection back to work.