Thanks for your feedback.

1. May I know how you connect the four units? The main Deco should be connected to the modem. What about the others? Are they all connected wirelessly?

Please note that DO NOT connect two Deco units to the modem directly as it may cause a network loop.

2. When it reminds no internet on the four units, please go back to the dashboard of the app. Then tap "More" -> "Advanced" -> "IPv4" -> "Internet Connection Type" and check if it could still obtain an IP address from the modem.

3. When the issue occurs, please disconnect the main Deco and connect a PC to the modem instead. Then check if internet access is still available in this case.

Furthermore, may I know your internet service provider? 

Thanks.

1. May I know how you connect the four units? The main Deco should be connected to the modem. What about the others? Are they all connected wirelessly?

Please note that DO NOT connect two Deco units to the modem directly as it may cause a network loop.

The primary one (which is the only one that stays connected cosistently) is hardwired to the modem.  The rest are connected to the system wirelessly.

2. When it reminds no internet on the four units, please go back to the dashboard of the app. Then tap "More" -> "Advanced" -> "IPv4" -> "Internet Connection Type" and check if it could still obtain an IP address from the modem.

This is not possible because it is in AP mode.  However, we have other wired devices connected to the modem (such as desktop computer) that are working without issue 24/7.  It is not an Internet connectivtiy issue.

3. When the issue occurs, please disconnect the main Deco and connect a PC to the modem instead. Then check if internet access is still available in this case.

See #2.

When the issue occurs, please unplug all the wireless Deco units and remain the main Deco connected to the modem only.

Then reboot the main Deco and check if it will recover.

If not, please disconnect the main Deco from the modem, then plug another PC to the same port on the modem and check if the PC could get internet access.

You can check the DHCP syslog on the modem as well. We guess may be the IP address pool of the modem is exhausted due to some unknown reason.

A factory reset of the entire system and resetup of the system seems to have fixed it.

I also had a period at the end of the password and removing that also seemed to increase stability of the wireless clients.

Strange on both fronts but I will keep my fingers crossed and update this thread if I have any more answers.