That method doesn't always work though, as routers that assign internal IP's dynamically (by DHCP) from a pool often have their lease time set to a non-unlimited time. Meaning that your router might hand out a new internal IP to your computer even though Windows is configured with a manual one that's expired (thus became invalid) to your router.

Best practice would be setting everything from your router. Look up DHCP, or whatever that's called in your specific router, and try finding a 'Reserve IP' setting. Look for things like IP adressing, My Devices, or static IP. You basically want to link an internal IP (those in the 192.168.x.x range) to your computer. Most routers make this easy by giving out a list of devices currently in use by their network name (such as GUS-PC or Android-xxxx) instead of MAC adress, with a button to make them static.

External IP's are never the problem. Yes they can change, theoretically rendering a progressing JJ2 game useless, but it rarely ever happens. They're usually dynamically assigned by your ISP and 'linked' to a customer, but some ISP's tend to release and re-assign all IP's as a rare maintenance measure.