R8000 Nighthawk X6 Connection Issues (with Virgin Media Superhub 3 in Modem Mode)

Hope someone can help me with my current plight. Not the most technical with Networking but will try to explain as best as I can.

We moved into a new house where our only viable ISP was Virgin Fibre, so we signed up and have a VM Superhub 3. Soon it became apparent that the signal from the Superhub was simply not good enough, devices upstairs would drop off the Wifi and with limited Mobile network in the house, it became very obvious that we needed a different solution (echoed by the teenagers in the house!)

So I bought a second-hand R8000 Nighthawk X6 Router. I then switched the Superhub into Modem Only mode and wired it via Ethernet to my Netgear Router. The better singal strength was appreciated round the house but still required an Extender so I have an EX7500 so wifi (2.4ghz and 5ghz x 2) signal strength is no longer an issue in our house.

However, what we experience is every now and then (maybe once a day) the Router seems to just die (physically everything looks on at the Router) but all devices drop the connection. So then I have to restart the Router. First few times, (for ease) I would restart the Router and the Superhub. The last time this happened, I restarted JUST the X6 Router and when it came back up, all devices reconnected and everything seemed to work again.

I guess my question is this, how do I continue to diagnose if there are ongoign issues with my Router? Are there settings that I may have enabled that may be causing my issues? I have the usual Parental Controls on, with Wifi and all three bands (1 x 2.4ghz and 2 x 5ghz). I use the OpenDNS servers for DNS (from a previous attempt to solve this issues). Trouble is I don't know where to start the diagnosis if the Router is faulty so any assistance is gratefully received.

Thanks in advance.

Phil