Occasionally VPN connection failed with below error messages:
Problem: 1) VPN Connection: The L2TP-VPN server did not respond...
It is suspected the root cause due to network device driver not being well managed by phone OS which shared with 4G and WIFI.
Resolution: Conduct below procedures in order to recover VPN connectivity as usual,
1) Android device:
1.1) Settings > Mobile Network > General settings >
18.104.22.168) Turn on Mobile Data (Off)
22.214.171.124) Turn on Mobile Data (On)
126.96.36.199) Retry Cowbee SDN-VPN connection
2) iPhone devices:
2.1) iPhone 7: Settings > Mobile Data >
188.8.131.52) Turn on Mobile Data (Off)
184.108.40.206) Turn on Mobile Data (On)
220.127.116.11) Retry Cowbee SDN-VPN connection
2.2) iPhone X: Settings > Cellular > Cellular Data (On/Off)
18.104.22.168) Turn on Cellular Data (Off)
22.214.171.124) Turn on Cellular Data (On)
126.96.36.199) Retry Cowbee SDN-VPN connection
Once it is turn (On / Off), please resume VPN connection with Cowbee SDN-VPN server again.
Alternatively, restart your phone device again to recover the service.