@Andre_BR It's basically an option that can be used if you are having trouble getting a DHCP address from your ISP. if you have it unchecked and are getting an address from your ISP you can leave it...

@pashvin Deco mesh is a TP-Link proprietary mesh technology and it's highly unlikely that they would do that. Check the difference between these mesh technologies here.

@Hobnobs Port forwarding and address reservation should not be dependent on the Deco WAN port configuration.

@Luxianos Make sure that you've plugged the T9UH in a USB 3 port for best performance.

@Messi9CR07 This could be due to an improper positioning of your range extender - follow this FAQ and check if you installed your RE properly.

@bhushan_ghag You FW upgrade file can be downloaded here.