Difficult and time consuming
This subject is too complicated and variable between different router models and manufacturers to supply a guide here. If you can not use DMZ or UPnP then google Port Forwarding and the make of your router to find information.
JRSM provides an easy way to get a list of the ports for each game. Simply click the List Ports button on each game tab to open a text file listing all the individual ports and ranges that need to be forwarded.
There is also use an excellent free tool called CurrPorts which can be helpful in identifying the ports a piece of software is using.
Once your device is accessible from the Internet we can install JustRace Server Manager (JRSM).