OpenVPN confusion

That is all about being an openVPN client, not a server.

If you need openVPN to your servers then you can follow the instructions here:

You will of course need accessible IP’s on the WAN links of your B3105G.

There is also no reason to use OpenVPN, a mac can connect to the L2TP server natively without extra software.

If your B3105G is only accessible via CGNAT then you will have to run a FusionHub and run the L2TP or OpenWan endpoint on your HUB. You can search for instructions about FusionHubs and VULTR and other cloud providers.