Wifi setup issues with my new Nighthawk AX4 Router

Help!!

I recently bought a Nighthawk AX4 router and a AX6000 Wifi Extender.

The installation has been a snap and the router seems to work perfectly for a very short period of time.

I am getting almost 100 Mega Bit Wifi speeds after setup and able to connect to the Wifi from multiple devices.

But then within an hour the Wifi connects start disappearing.

Password shows up as not correct or no internet is found message comes up.

Then the wifi name no longer shows on the active wifi options to choose from.

But, if you look at the Wifi router all the lights are green - everything shows as normal?

Even connecting using a RJ45 cable does not seem to work in some cases.

If I reset the router to factory resets and then start the install all over again it works great and installs perfectly.

But then an hour later same things seems to happen.

I have tried selecting "Channel 4" vs auto per a video I saw on trouble shooting - but same issue - works great then goes away?

I have reinstalled router 4 times now - keep all factory setting and changing them - same result.

Any ideas what is going on here?

We use Google Fiber - so have extremely good internet if connected directly using a cable.

But our home is almost 100 years old and the plaster walls made wifi signals difficult so we bought another router for the upstairs.