I tried lots of things, but in the end, it looks like it was the IPV4 Gateway and Custom DNS Server needed to be the same as my Router's IP. As soon as I'd set those, mining resumed.
How it ever worked in the first place, I have no idea, as these were not settings I changed when first setting up the miner. Ghost in the machine?!?
Eventhough it's a antminer, it's still cheap produced Chinese shizzle