If the WAN IP address your router reports is in the 100.64.0.0/10 (100.64.0.0 - 100.127.255.255) network, then your ISP uses carrier grade NAT (CG-NAT). CG-NAT works like the NAT on your home router, except it is maintained by your ISP. It is a means of grouping many customers behind a relatively small number of public IP addresses (that 77.x.x.x address is the IP on the other side of the CG-NAT). This places you in a double-NAT situation, which you have no ability to correct by any configuration of your equipment.
Unfortunately, you have very few options here if your carrier does not allow you to pay for a public IP address. This may change when Plex implements end-to-end IPv6 support for remote access.
