Occasionally VPN connection failed with below error messages:
Problem: 1) VPN Connection: The L2TP-VPN server did not respond...



Cause:
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 >
1.1.1.1) Turn on Mobile Data (Off)
1.1.1.2) Turn on Mobile Data (On)
1.1.1.3) Retry Cowbee SDN-VPN connection

2) iPhone devices:
2.1) iPhone 7: Settings > Mobile Data >
2.1.1.1) Turn on Mobile Data (Off)
2.1.1.2) Turn on Mobile Data (On)
2.1.1.3) Retry Cowbee SDN-VPN connection

2.2) iPhone X: Settings > Cellular > Cellular Data (On/Off)
2.2.1.1) Turn on Cellular Data (Off)
2.2.1.2) Turn on Cellular Data (On)
2.2.1.3) 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.

Thank you!