So Untangle is setup as the default gateway at 192.168.100.1/24, it is the authorative DHCP server issuing addresses from 192.168.100.100 to 192.168.100.200 and is successfully connected to the Internet. Untangle uses OpenVPN for remote access. Accessing the VPN gives me the address 192.168.40.5.

However, I cannot ping any machines on the internal 192.168.100.x network remotely. Clearly, there is something basic that I am missing. What is it and how is it solved?

Update:

The VPN was not setup with the internal network. Since Untangle only allows editing the VPN setup once, the VPN had to be removed and reinstalled with the internal network exported. Now it works.

The lesson is that the internal network must be setup before configuring the VPN.