The max rate of upstream and downstream seems normal and quite close to speed given by your ISP. You are suggested to report the problem here for further help....

Connect the mian router to one of LAN ports of W8961. Change its LAN IP to be in the same subnet as the main network. Disable DHCP.

Uninstall the driver. Uninstall anti-virus software and disable Windows firewall. Download the driver for Windows 8.1 and install the driver manually....

What's the hardware version and firmware version of your C7? Do you need a username and password to access it? Did you enable write access or set storage authority as read and write?

How often doees it happen? How many wireless clients in total connecting to the Wi-Fi network at the same time? If you just connect one or two devices, does it occur as well? Try to change transmit...

How about upstream and downstream on Status page of W9980? The speed 40/10 is tested by cable connection or wireless connection. How did you run the test?