Interesting I have just been getting exactly the same issue and I am on fibre with plusnet, if I loose connection or reboot it syncs up with the line fine but just doesn't authenticate, I rerun quick setup and it is fine although I had to run it twice today. The quick setup does retain my username and password though!
The reason I have to reboot is I keep loosing the ability to FTP to my website, it will just stop working and rebooting fixes it!
Anyway this is the second 9980 I have had the first one started randomly locking up so Amazon sent me a replacement.
Anyhow I have e-mailed tp-link and will post the outcome.