VPN client connect to another container in the same Docker network with the VPN server

Hi. I’m building a VPN tunnel with a VPN server running on a Docker container on top of an Ubuntu server. I have other containers running in the same Docker network of the VPN server container. I wanted to use firewall so only VPN clients can have access to the other containers in the same Docker network with the VPN server container. Which IP address or subnet should I allow in the firewall? I tried allowing the public IP of the Ubuntu server but still can’t access the other container somehow. I’d appreciate if anyone can help or give hints about this matter. Thank you!