This is happening because both computers, PC and server, are connected to a router which is doing network address translation
"Network Address Translation (NAT) is a network protocol used in IPv4 networks that allows multiple devices to connect to a public network using the same public IPv4 address. NAT was originally designed in an attempt to help conserve IPv4 addresses." (Wikipedia)
This can make it difficult for servers to accept connections from the outside world. If you want your server to be accessible from outside your local network, I suggest you look up how to do 'port forwarding' on your router.