To get the maximum speed, it would be better to use C7 as an access point as it is a NAT device when it is a router. When connected with wan port, did you test the speed on clients connected to its...

Did you ever try to connect the printer to any other Wi-Fi networks? Change the SSID and password for C9 first. And try to reconnect via wireless config tool. If you get the same error, try to...

On Device Info page, what can you see? :)

The router should reconnect automatically as WAN connection type is dynamic IP. Connect the modem directly to your PC instead of WAN2. Does it disconnect and reconnect automatically?

I replied you in another thread.

The PC is running in Windows 7? Download the chipmaker's driver AR9271 for WN722NC, https://www.atheros.cz/atheros-wireless-drivers.html Try to install the driver manually....