Good afternoon

im looking for some help/advise if possible im new to all of this and learning along the way 

i have been given a project so my customer can see all his CCTV systems around the country from one location 

so i have decided to host a vpn server on my er605 which has a static ip address and using openvpn server on the router

on each site there is a 4g router and a er605 the ER is the open vpn client and a nvr (a fixed ip sim is not possible due to cost and ddns does not work on 4g due to been cgnat )

i have managed to get 2 sites connected to my open vpn server the problem im having is that the server end can see both devices but i cannot get each client to see each other which i need is there any way of making this happen

thank you