Unable to connect Apple Bluetooth Keyboard A1314 to Mac mini

I am trying to pair an Apple Bluetooth keyboard A1314, bought second hand, to my Mac mini 2024 M4 Pro running Tahoe 26.6.2.

1. I put fresh batteries into the A1314, then held the power button on for about 6 seconds. The green light appeared solid at first then changed to two flashes followed by a brief pause, this flash pattern then kept repeating.
2. In Bluetooth settings, Nearby Devices, the keyboard appeared as System Administrator's Keyboard (presumably named by its previous owner).
3. Still holding the power button on, I clicked on Connect. A dialog box appeared with a 6 digit code, advising me to enter the code on the keyboard and press Return. So I did that.
4. I tested the keyboard and nothing appeared on my screen. It was not connected.
5. In Bluetooth Settings, the keyboard was listed and the Connect button appeared next to it. I repeated step 3 many times with the same result.

How do I connect the keyboard?

Is there a way to reset it? (note I bought it without batteries, so I installed fresh batteries when I brought it home).

Thanks in advance for your help with this.