@siens TP-Link uses self-signed certificates - ther's nothing to download. If you already configured HTTPS access, use this guide for accessing the router's WebGUI. If you want to enable WebGUI HTTPS...

@Matt_V Remove the ethernet cable from AX23 LAN1 port and plug it into the WAN port (blue port). Then everything should be fine.

@Veryyconfused Make sure that you're with the latest firmware version on your WA850RE - from here. If not, download it and update manually - here's how.

@Dyspo Check MR600 user guide - Chapter 10.2 for port-forwarding using virtual servers. For DMZ - Chapter 10.4.

@Dyspo To avoid double NAT configure WR840 in AP mode using this guide. Regarding NAT type 1 check this video. It would be probably enough to open the proper portts on MR600 to play normaly in...

@MarcusMarcus You can do the following experiment: Unplug the RE450 and move it near your main router. Plug it back and connect the ethernet cable directly to your main router LAN port. Check the...