Alternative to the function Force VPN (No Internet if VPN is not connected) on AR750S

Actually, that is exactly how I understood it!
And you are correct, in the new firmware their is no way to force VPN (ie. kill internet) if the VPN fails or disconnects.
This is exactly the same problem that I reported back in July 17 with the early v2 firmware (which was fixed then within 24 hours IIRC):

Glitch