The modem’s “WAN IP address” is a private network. That means your ISP isn’t assigning a publicly routable IP4v address but placed you in a “CGNAT”. That means Plex has no means to connect to your server from outside your home network.
As the ISP is assigning the WAN IP address to your router, configuring a port forward on the router won’t be enough. Your best shot is asking your ISP if they can assign you a publicly routable IPv4 address. Some ISPs will consider this. Most of those will try to sell you a static IP address – usually for a steep price. You don’t need a static IP address! Although with some ISPs that’s the only option to get a publicly routable IPv4 address… so that’ll be up to you…
