Check which one is the IP address of your Mac and your Public IP, for example going to https://www.whatismyip.com/
If they are different, your internet router is doing NAT. You have to configure in the NAT section of the router a rule to forward everything coming to a specific port to your Mac.
For example, you can create a rule to forward anything coming to port 6002 to your MAC
The external server should point to the Public IP found before and the 6002 port.
Keep in mind that anybody sending to 6002 will be able to reach your MAC, so maybe the rule should filter also the Source IP to permit access only to traffic coming from the external server