Hi guys, wondering if someone can help

Im in the UK with BT Fibre broadband (70mb download - 30mb upload)
I have a unibox hd and openwebif
I set up a ddns account on no-ip
I set up port forwarding (port 8096 - port 80) on my router along with setting the DDNS and placed the unibox in the DMZ.

It all works great for around an hour, then everything comes to a halt. I am unable to get online, or even access my routers settings. I have to unplug the lan cable to my unibox to access the web again and the routers settings.
If I plug the unibox back in the network crashes again immediately. I have to remove my DDNS , DMZ and port forwarding settings to correct this.

What am I doing wrong? Why does it work ok for 1 hour roughly then stop?

The F1 season starts this weekend and I don't want to miss the first race

Thank you