TP-Link Deco M5 hard factory reset
There's a little recessed reset button underneath the unit. However, pressing it doesn't always appear to actually reset it 🤦. The instructions are vague … is it one second or ten? What colour should the lights be when the factory reset is attempted?
Having spent the last hour messing around with these things (to change the network they were connected to), the most successful approach appears to involve waiting until you get a blue flashing light, and then press the reset button for about 10 seconds. The light turns yellow, and the unit reboots, when the blue flashing starts up again you should be able to connect to the unit using the Deco app.
My interpretation is that the unit boots, tries to connect to the old network, and — once it's ready to connect — only then becomes responsive to the reset button. But, what do I know?
What a palaver.