GTA5 NAT
You need to understand the difference between WAN and LAN. WAN (Wide Area Network) is in simple terms 'the internet'. LAN (Local Area Network) is the network in your house. It's important for you to realise that they are two separate things. Data packets from the Internet never reach your LAN. What actually happens when a packet arrives from the Internet is that your router reads it, works out which machine on your LAN it relates to and then creates a new LAN packet that contains the data and sends that into your LAN.
This is called Network Address Translation - you can look that up separately.
The important take-away point here is that no-one on the internet can access your PC directly. It simply isn't possible.
Now normally your router can work out what LAN address to send a packet to because the packet header has a conversation ID and the router tracks all the conversations started by your devices. It knows that 'conversation XX' packets should be sent to YYY.YYY.YYY.YYY because it was YYY.YYY.YYY.YYY that started the conversation. But if a random packet arrives it has no clue who to send it to so it just discards it. What you need to do is tell your router that certain packets should always go to a specific address.
This is nearly always done by port number. You create a port forwarding rule on your router. What this rule says is 'If you ever receive a packet for port XXX forward it to that port on the device at yyy.yyy.yyy.yyy'. As for how you do that it depends on the router. Most of them refer to it as port forwarding but some call it 'virtual server'. They are basically the same thing but virtual server allows you to specify a different target port so the rule can be 'If you ever receive a packet for port XXX forward it to port ZZZ on the device at YYY.YYY.YYY.YYY'.
The other thing you will have to do is ensure that your target PC has a fixed LAN address. That is usually done by going into its network settings and manually setting the IP address. You should pick an address that is outside of your router's 'DHCP pool'. If you get stuck on that ask here again
This is called Network Address Translation - you can look that up separately.
The important take-away point here is that no-one on the internet can access your PC directly. It simply isn't possible.
Now normally your router can work out what LAN address to send a packet to because the packet header has a conversation ID and the router tracks all the conversations started by your devices. It knows that 'conversation XX' packets should be sent to YYY.YYY.YYY.YYY because it was YYY.YYY.YYY.YYY that started the conversation. But if a random packet arrives it has no clue who to send it to so it just discards it. What you need to do is tell your router that certain packets should always go to a specific address.
This is nearly always done by port number. You create a port forwarding rule on your router. What this rule says is 'If you ever receive a packet for port XXX forward it to that port on the device at yyy.yyy.yyy.yyy'. As for how you do that it depends on the router. Most of them refer to it as port forwarding but some call it 'virtual server'. They are basically the same thing but virtual server allows you to specify a different target port so the rule can be 'If you ever receive a packet for port XXX forward it to port ZZZ on the device at YYY.YYY.YYY.YYY'.
The other thing you will have to do is ensure that your target PC has a fixed LAN address. That is usually done by going into its network settings and manually setting the IP address. You should pick an address that is outside of your router's 'DHCP pool'. If you get stuck on that ask here again