There are various reasons which may cause the VPN to disconnect. Some of the solutions are listed below:
1- Switching the Protocol: Disconnection issues can be solved by switching in between the protocols, i.e. PPTP, L2TP and OpenVPN.
2- Firewall: If switching the protocols doesn’t help then check Router/ Windows/3rd Party Firewall settings. First, temporarily disable your firewalls and try again. Sometimes router firewalls cause the disconnection problems because Wi-Fi routers usually grind the VPN to a stop after a few minutes of use, plainly because they can’t keep up with SPI / Firewall turned on.
3- Internet Connection: If you have tried every technical aspect that has been discussed above and still facing the disconnection issue, then you may finally want to check your internet connection and the specific settings that have been provided by your ISP, so as to ensure that from which end are you encountering the issue.