Not sure, but you'd better make a backup before you upgrade it.

So you got two TP-LINK devices, one TD-8816 and one Archer C7, right? Is your C7 running in latest firmware?

You may downgrade the old firmware to test if it works OK again.

Your laptops can detect other Wi-Fi networks and connect OK? Change the SSID and try a different channel.

Connecting to W8961ND by cable is not stable but wireless connection to it works OK? When wired connection drops, what's the status of local area connection on your PC? Can you login the web...

Go ahead. It won't brick the device. Ensure you upgrade it by cable connection without power cut-off.