@Kevin_Z  Exact same issue as the others in this thread: https://community.tp-link.com/en/home/forum/topic/177276. Deco P9 system is running in access point mode. Internet from ISP works #1, all devices wired are working correctly, but the Deco P9 stops detecting internet and wireless devices lose connectivity, but still shows Wifi as up on the device, so all devices are still connected to wifi from Deco and cannot connect to the internet or local network. After few minutes, the internet comes back on the devices. Deco P9 devices are the only one providing WIFI connection inside the local network.

Answers to your questions:
1. The current network diagram. And the operation mode of the Deco.
== Operation Mode: Access Point Mode
== Network topology
===== ISP Modem ------ Ethernet cable ------ FreshTomato Router (WAN Port) (firmware version 2020.02)
===== FreshTomato Router (LAN Port 1) ------ Ethernet cable ------ network switch ------ Ethernet cable  ------ Deco P9-A (main) ------ Powerline ------ Deco P9-B (slave)
===== FreshTomato Router (LAN Port 2) ------ Ethernet cable ------ network switch ------ Ethernet cable  ----- Deco P9-C (slave)
== Note 1: All P9 devices are connected in a socket directly on the wall, no powerbar involved.
== Note 2: 20 devices are connected according to the Deco app.

2. The led status of Deco when it stops working. And whether you can access internet from the modem when Deco stops working. 
== The Deco P9-A (main) is 6 feet away from the devices connected wirelessly (DEV-1, DEV-2).

== When WIFI internet connection is gone, The Deco P9-A, P9-B and P9-C are having a LED which is solid white 
== When WIFI internet connection is gone, the wired devices (DEV-3, DEV-4) are still working perfectly in the network
== When WIFI internet connection is gone, cannot reach local IP addresses as well (ex: router) from device DEV-1
== When WIFI internet connection is gone, disabling the WIFI and enabling it on the device DEV-1 and DEV-2 fixes the problem, it reconnects and internet is back on the device.
== In the Deco app, all P9 devices are showing as online.

== Before my upgrade to the Deco P9, I was using mixed access point and never had this kind of issue.

3. Try to swap the location of the Deco units and you can switch the slave Deco to the main Deco to give it a shot. 
https://www.tp-link.com/support/faq/2400/
== Cannot do the main switch from the Deco application, even if I am connected on the WIFI network of the deco, the option doesn't show in the application when pressing the three dots icon.
== Did not try to physically switch the Deco P9-A and P9-C yet, I will wait for your first investigation result first.

4. You can assign a static IP for the clients or reserve an IP address on the Deco app. 
https://www.tp-link.com/support/faq/1795/
== All wired and wireless clients have static reserved IP addresses using MAC address, including the Deco P9 devices. This is configured in the FreshTomato router.

5. Please ensure that the firmware is up to date.
== Firmware version: 1.0.2 Build 20190917 Rel. 71168, Deco app says it's the latest version. Hardware version: V1 (US)

Note:

- I don't mind enabling some advanced logging if it helps, if such thing is available.

- Fast roaming is not enabled.