So using FortiClient and having disconnects implies users are remote and connecting to VPN. I am making this assumption that the VPN connection is terminating and disconnecting users.
If that is correct, you have to understand that if the user can still access the internet after disconnecting from VPN, that just tells you *their* internet is fine.
What we need to focus on is the internet link on your FortiGate that is servicing the VPN clients. As soon as clients disconnect they will no longer be relying on the internet that is connected to your FortiGate.
So, how are you confirming the FortiGate's internet connection is stable?