Yes my ISP does not assign me a fixed IPv4. It is stated on their website. They do, however, assign me fixed IPv6. Unfortunately I only have a public IPv4 that is shared with other home routers (other customers in the building probably), and my home router is not directly connected to the Internet, but only connect to a layer of subnetwork with the internal IP (started with 10.x.x.x). So now I have no choice but to use IPv6 protocal for OpenVPN server.
@alzhao What version of firmware supports ipv6 OpenVPN server? and How do I upgrade to it?