Smart HomeKit accessories Not Responding some time after activating xFi Gateway

Last night I removed my old router (OnHub wireless router from Google) and cable modem (a Motorola SURFboard SB6141). My Home app has a lot of accessories setup (lots of light switches, smart plug, garage opener, video camera), all of which were working fine for weeks without any hiccups.

I then ordered a xFi Gateway from Comcast to replace my router + modem and get faster Internet.

I unplugged my old router and modem and connected my new xFi Gateway (white) that I received from Comcast. After activating it, I named the wifi network the same as the one on my old router and same password. That seemed to work great as all computer, Apple TVs, phones, apple watches and all smart devices were connected and working. I made sure my smart HomeKit accessories were all working (garage door opener, light switches, cameras, etc.)

Then I went to bed and when I got up this morning I noticed that a light at the front of the house which was scheduled to turn off at sunrise was on. Then I went into the Home app and I noticed that all the accessories show as "Not Responding". 

I tried reconfiguring a smart plug that I have for a toaster in the kitchen and I was successful but after some time it now shows "Not Responding".

My xFi Gateway assigns IP addresses starting with 10.0.0.2. I don't recall whether my OnHub router used that range.

I went in my Home app's Home Settings and under Hubs & Bridges I see a HomePod in Connected status, which is good because this is the one closest to the xFi Gateway and where the wifi signal is the strongest. I also see four Apple TVs and one HomePod mini all in Standby.

I have also power cycled my xFi Gateway. The problem persists, all accessories show "Not Responding".

I have not tried any drastic things such as Remove Home from the Home app.

Any advise or things to check?

Thank you.