@HawkCraft,

Any kind of setup that is required will be done on the Main router for the network, as this will be the device controlling the ports. The Range Extender is simply copying the signal and then broadcasting it.

I will warn you that, as a result of the nature of Range Extenders, you may have a suboptimal experience because of how an RE communicates with the router.

Also, in terms of your Minecraft server, are you trying to make the server accessible to people outside of your network, or simply from devices connected to your home network? If you are not looking to have players from the internet join, you may not even need to change any ports.

  @HawkCraft,

If you set the ports on your router already, there is some disconnect that is preventing your router from correctly identifying your PC as the IP that needs the ports. The range extender will simply forward the signal, so it doesn't need to have any ports configured. Instead, I believe that the Range Extender is managing the devices(not connections) and sending an alternative MAC address.

Looks like another user has had almost the exact same question a while ago, and one of my coworkers answered it pretty thoroughly: Using port forwarding through a RE450 wifi extender

It looks like you have to find your device in the router settings by MAC address, rather than using the MAC address normally used for the PC