@Psonbre 

The extender does not need port forwarding. If the extender was actually a router, then you would do double port forwarding, but not this case. What needs to be done is you need to find out what the IP of the client device is. When a device connects through our extender it gets a different MAC address so your router is probably giving a different IP compared to the device being directly connected to the router. The first half of the MAC address should be the same, and the other half is what will be different.

You can also use the Tether app to look at the connected devices' IP info or do an IP lookup on the device itself.

Once you have the IP of your server, do the port forwarding in your main router.

@Tony 
Thanks a lot! I managed to make the server go online when I'm connected to the extender, however it seems to only work when my computer is connected to the extender's wifi, but not when it is connected by cable. When using the tether app, I can only see my computer in the clients when it's connected by wifi and not by cable. Since my internet is pretty slow without using a cable I'd like to know if it's possible to make it work with it. 

Thanks again!

@Psonbre I am having the same need to connect my device (Helium miner) by ehternet because wifi signal is too low.  Did you get it to work?

  @Psonbre Me too, COVRX-1870s extending my DIR-X5460, clients do not show connected when connected by cable, which I need to be for my home office/ printers etc...

Port forwards and DMZ from main X5460 simply don't work. And I don't show as a connected device to 1870, nor do the 1870's show up as extenders to the 5460 but they show as connected clients! However in the D Link Wifi app they show as Extenders!

Even with a ton of networking experience these extenders are doing my head in. 

  @Psonbre ay bro I also want to host my Minecraft Server so my friends can join and could you please tell me what did you finally put into the LAN IP Address when Portforwarding in your Routers settings? 

  @FELLOWMAN Honestly i gave up lol... I decided to use aternos instead. It's free and it works pretty well. There are some downsides though, you have to launch the server every time you want to play which can take some time and it closes itself after five minutes of being empty. The servers can be pretty laggy too. However it's 100% free and you can make it so any of your friends can launch the server when they want to play. They also have a pretty decent number of mods and plugins you can use. Hope this helps !

  @Psonbre Yea hosting a Minecraft server is too hard in my opinion and we've also used Minehut and Aternos before but after playing for a while and your world get bigger it starts to lag pretty much and it's not funny anymore.