AC790s keeps rebooting

Hello, my AC790s (BigPond) worked fine until I tried to update the software. Since then it constantly reboots and only show the startpage (BigPond logo). To solve that problem I did a hard reset (long press of the start button). Now it tries to start 2 or 3 times (showing the startpage again) and after that it show that it's installing an update. It then stops updating at about 10 to 20% and restarts the update process instantly.

I'm not able to enter the homescreen or any menu at all, so I don't know the current firmware or connect it to the internet.

Is there anyone who can help me solve this problem?

Thanks in advance

Best regards

Klemens