OpenVPN routing between sim cards ant the internet

We want to setup an OpenVPN that will be connected to the internet on one side and over VPN to a mobile operator on the other side. The sim cards receive a private internal ip from the operator. The routers in which these sim cards are placed have an OpenVPN functionality already integrated. We want all traffic from the sim cards to go over the VPN towards the OpenVPN and from there on the internet. The main functionality is that the sim cards must be accessible from the internet on predefined ports ! Of course Dyndns or other functionality to achieve this is an option. Should not be a major job for someone with OpenVPn capabilities.