Hi PhilS

Yes, enable/disable the VPN in the GLI GUI always works fine.

Also I checked many times physical switch to enable/disable the VPN and didn’t have any problem.

Disconnection is possible if connecting 3G-modem in a situation when scheme WiFi->VPN->WiFi is active, and vice versa.
In such situations, you should disable and then enable the VPN again. Besides, IMHO should avoid situations with two connections to the Internet simultaneously (WiFi as client + 3G-modem).