Since my last post got a best answer AND was deleted I'll just post the steps from Nokia itself:
On your Lumia phone, delete the non-functioning WLAN access point (internet connection) and connect it again:
Disconnect the WLAN connection if connected.
Press Start, then swipe left to the App list.
Select Settings > WiFi.
Press Advanced at the bottom of the screen.
Tap and hold on the access point name and select Delete.
Now add the access point again.
On the Advanced screen, tap Add, type the network name in the Network name box, and then tap Add.
On the Sign in screen, type the password in the Password box, and then tap Done.
Tap the Back key once finished.