You need to use a direct Ethernet cable or Powerline adapters if you want a reliable connection. Wireless is subject to too much interference.
Port forwarding need to be done the way I have described, otherwise it can be unreliable.
There are other options discussed on my link which I posted. Even a static IP address may help.
There are lots of report of problems with the wireless on the HH3. If your old router is not ISP locked, then you could try that?
Apart from that, you could try looking at the suggestions on this link.
Otherwise you will just have to put up with the wireless disconnections.
I will look at that link when i get home, due to being restricted from help while not connected to the WIFI im having problems with.
EDIT: I will try and get a long Ethernet cable and connect it temporary to test maybe that will give us a better view on whats happening.