Remotely accessing the secondary router in a double NAT topology
In this setup the 'HTTP referer Head Check' feature on Archer C64 (secondary router) is used to make this possible.
This setup should work when Archer C64 (secondary router) is set either in wireless router or AP mode.
The first step is to configure the remote access on the secondary router (Archer C64) :
Note: Make sure that you've unchecked the HTTP Referer Head Check otherwise this won't work.
The second step is to configure a port forwarding on the main router (Archer C4000) so the secondary (Archer C64) can be accessed on its remote access port:
Here the IP address 192.168.1.10 is the WAN IP address of C64 router (or its LAN IP address if it's configured in AP mode).
Note: Make sure that you have your public IP address on the C4000 WAN (Internet) interface otherwise that won't work.
Now from an external network (Internet) you should be able to access your Archer C64 router WebGUI at the following URL: http://<your public IP address>:8080