Too bad about the Netgear. On official firmware? I don’t know enough about them to know whether it even supports two configurations at the same time or if you have to go to ddwrt or tomato.
Did you try the original Windows configuration to see if it worked on the 750?
You might see if any of this helps: GL.iNet 750S Slate Router OpenVPN 'TAP' client to Netgear NightHawk OpenVPN server or this: Netgear R8500 OpenVPN Server "Can't Connect using GL-AR750 as OpenVPN Client"