Further many PoE switches are smart or managed and you can turn the PoE power off at the switch. It might be that a firmware update on the switch has put the port back to default which is not to provide and PoE power.

IMHO dumb PoE switches are just that dumb as power cycling a device requires mucking about unplugging and plugging ethernet cables.

A laptop doesn't usually require (or use) PoE and so doesn't test if the port is providing PoE it just tests if it is providing data.

As far as PSUs if you check the requirements for the device then there are many universal adapters that may well provide what is needed for testing.

Indeed, I use Ubiquiti switches and I can not only remotely switch the power on/ff to each port, I can see how much power is being drawn by each device. Very useful!

I found a plug that fits the TP-Link and there's still no power.

Maybe it is broken.

So weird.

There is one LED which comes on when there's power going to it.
But there's nothing when plugged into Ethernet, the mains or both.

I'm starting to think that it is broken

How long have you had it and how long was the warranty?

Couple of years.
It had Lifetime warranty but TP-Link has refused to provide a new one because I don't have any proof of purchase. I don't have the invoice / payment receipt or anything like that.
The item was provided by a bloke who did the electrics in the house. We didn't keep the invoice (why would we?) and he has since passed away.
He was a sole trader so ... don't think I can get a copy of the invoice.

Oh well.