Just because it came in handy for me, here’s how to factory reset a Brother HL-2170W.

1. Turn the printer off.
2. Turn the printer on while holding the GO button down.
3. All lights should be off, now push the GO button 7 times, it will flash each time.
4. Wait for a minute. The printer will start it's reboot sequence.

We are in temporary accommodation and the printer was still setup on our old network and I couldn’t get into it, problem now solved.

After the reset I found the printer by looking in our router’s DHCP Lease table and set a static IP address for it’s MAC address, but that’s all easy once it’s on the network :-)