Hitron: Strict NAT

You said your OLD Xbox had some ports open? did you try opening ports on the NEW Xbox?

Not sure what links you have tried but if you haven't already tried this one give it a shot. https://support.xbox.com/en-US/xbox-one/networking/upnp-not-successful

If that doesn't work try performing a hard reset EXACTLY like this. Hold the power button on the console for 8 seconds or until the power shuts down completely. Unplug the power cord from the back of the console for 5 minutes. While the console
is off unplug your router for 3 to for minutes the plug the power cords back in let the router reset and then restart the console.

Hope this helps. Good Luck.

When I first got my Xbox One S I had issues with it connecting to my Hitron router as well. The problem is more than likely the router, as it was more than likely provided by your ISP. The biggest issue I have found with the Hitron is that it can not support
very many devices all connected at the same time. Which results in loss of connection, slow connections, and Moderate or Strict NAT Types and UPnP being unavailable. Believe me, when I say this I but I really don't think it's your Xbox. I sent mine in for
the exact same reason and it got sent back the day they opened it and tested it.

Try this:-
www.unofficialguidetolive.co.uk/.../62-my-nat-is-strict-or-moderate

Or this:-
www.unofficialguidetolive.co.uk/.../62-my-nat-is-strict-or-moderate

Make sure you haven't got a strict nat type here!
support.xbox.com/.../nat-type-strict