It looks like ROS openvpn server is using net30 topology and we cannot change that.
Reference: https://community.openvpn.net/openvpn/wiki/Topology
For this to work, the openvpn server has to handout to the client a pair of ip addresses of the same /30 subnet to setup the tun device in a point-to-point like setup.
My doubt is how is the intended configuration to achieve this through a ppp profile instead of a specific configuration per ppp secret. On the profile we can setup an IP pool but from my experience the openvpn server is not taking care of using ip addresses of the poool within the same /30 subnet per client.
On a sidenote, bringing openvpn support up to current millenium standards (udp, lzo , topology) would be a welcome addition.
BR!