I've been through the process numerous times. I choose Settings > Network and Internet > Show available networks. I then choose the "printer" and provide the network security key (cut and paste; same as used for 2.4 and 5Ghz networks) then choose Next then choose Yes to allow to be discoverable. It consistently returns "Network Security Key isn't correct". I happen to have a Network Configuration data sheet printed by holding down the printer's ON button for 10 seconds. I printed it while following other instructions. As it turns out, the SSID contains the name of a previously used network name, Niko, that no longer exists. 3 lines below it, for Wireless Link Status, I see Failed to associate.
This indicates to me that the printer's firmware has the incorrect value, Niko, but I can't find any way to change it.
A few days ago, while trying to resolve the printer problem I thought it might be best to remove it from Windows device manager. I did so, now I can't add it back using software provided by Brother (presumably the same as that which came on CD when the printer was new, yeas ago).
If anyone can tell me how to fix the problem or where to find a solution, I'd be very much obliged.


