iMac Magic Keyboard (A1644) displays symbols/characters instead of letters when typing

Am currently using a 3rd party backlit, Bluetooth keyboard. 

Only use this Magic Keyboard that came with my 2015 iMac every now and then. 

Recently got a Mac mini and tried to use it again and experienced the following: 

It is detected and pairs successfully, but displays symbols and characters instead of the letters on my keyboard when typing. Tried contacting Apple Support and was advised to do a [1] Factory Reset (Shift-Option-Clicking Bluetooth icon in Menu bar) as shown in the pictures. Removed and re-paired the keyboard but to no avail.

Support officer also talked me through a [2] Resetting NVRAM and [3]deleting keyboard .plist files in xx > Library > Preferences folder. Nothing worked so far. 

Input sources in Keyboard pane of System Preferences is ABC / U.S International.

Is there any way to re-flash the firmware or fix this keyboard still or is it a goner?