Laptop disconnecting from wifi, only when "extender" is connected

I have one laptop (HP) that won't stay connected to wifi. It worked yesterday until I installed a new router/modem. All other devices unaffected. Here's my new setup:
New Arris SBG7400AC2 router/modem connected by coax. 2.4 and 5 have channel auto. SSIDs: Work, Work5G
DHCP range: 192.168.0.2-192.168.0.249
IP (gateway): 192.168.0.1 255.255.255.0

Older Netgear C6300 connected to Arris (wired). Turned off Use Gateway as DHCP Server
2.4 channel is auto and 5 is set to 157. SSIDs: Amp, Amp.5G
IP: 192.168.0.250 255.255.255.0

HP IP currently 192.168.0.4 (obtained auto)

With all equipment, I can ping both IPs successfully. With HP, I can ping .250 successfully, every time. I fail 3or4 out of 4 times (request timed out) when pinging .1, with HP.

I was thinking, must be internal NIC or settings on HP. So, I tried ipconfig release, renew, flushdns, various netsh commands (i i r r, winsock reset, etc.), making sure computer can't power down wifi, made sure on private network, forget network, turned on connect automatically (to each SSID individually, one at a time), lowered roam to Medium-low (so wifi wouldn't try to bounce from one SSID to another), turned off Hotspot 2.0 networks, set obtain DNS automatically and tried using Google's DNS (8.8.8.8, 8.8.4.4) and the list goes on. Ran windows network diagnostics. Performed Network reset. Uninstalled, reinstalled, and rolled back drivers. Even ran AVG full/deep scan. Rebooted after each step.

I've unplugged, waited 30 seconds, plugged in both Arris and Netgear. I went to Spectrum web site and did a reset that way.

Here's the kicker; the HP is fine, if I disconnect power from Netgear! I can connect to Work or Work5G and browse away like nothing is ever wrong. With HP, I can ping .250 successfully, every time AND I can ping .1, with HP successfully every time.

As soon as the Netgear is powered up, the HP connects, allows me to browse slowly for about 1 minute, then I get server not found errors, can't reach this page, etc. using Chrome, Edge and Firefox.

All other devices use the Amp or Amp.5G to connect, as they had in the past, so I never had to reconfigure any other devices.

I'm at my wit's end. Any help, suggestions or configs you can suggest, would be much appreciated. The HP has no Ethernet port, so a wired connection isn't possible.

I'm using the netgear to extend the range/coverage of wifi to my home. The Arris is in one corner of the house (only coax connected to Spectrum). The netgear is being used to provide better coverage. Also, having netgear set up, kept me from having to set all my devices (for which there are many) to use the new Work SSID. The extra ports don't hurt either as I have 3 out of 4 ports used on both routers, due to security wiring, as well.

Re: DHCP is off and no guest network is enabled. Of course the 2.4 and 5Ghz are on (wireless AP), as I've mentioned for SSIDs Amp and Amp.5G, for all previously set up devices to connect seamlessly.

Also, I looked at wired and active client list and .1 wasn't listed anywhere.

PS I used Netgear diagnostics to ping HP and it was successful each time.

- - - Updated - - -

Solution
Uninstalled AVG virus software
Sigh

flayer6- Being network illiterate such as I am your dilemma caught my eye. I had about the same problems with an Arris Modem/Router and (3) GoCoax adapters on a closed MoCA system. At least for me they all were tripping over the 192.168.254.254.IP When the MoCA was connected to the router the (W-10) computer wouldn't receive over ethernet and wi-fi link would drop out if even (intermittently) established, still three wi-fi computers on Linux would work.. Disconnected MoCA to router and W-10 computer wi-fi would function normally. It is explained in the link below and all works great now.

Hope this helps

Windows 10 desktop keeps intermittently dropping wifi