Factory reset seemed to be successful.
I did a factory reset by pressing and holding the button on the back. I was expecting a reset after about 10 seconds but nothing happened so I probably held it down for 30 seconds and as soon as I released it did the reset.
After the 'factory reset' my WiFi name and password were left unchanged so I was a bit skeptical that it worked. However, my port-forwards had been deleted. I re-added the port-forwards (for Plex).
Since then I have not seen any messages for the WiFi dropping off which I believe was caused by the DNS error and I have not had any DNS errors on my main computer and I have not seen any delays when trying to access various web sites after waking up my computer.