@TomorrowV 

My main router from the ISP is set to Bridge mode, and Deco is connected as the primary router

and

Deco S7 is configured as an Access Point (not Router Mode)

These are two conflicting statements. Please clarify.

  @Alexandre. 

Thank you for your response. I understand how my description may have caused confusion. Let me clarify:

1. My ISP router is set to Bridge Mode, meaning it simply passes the internet connection to Deco.

2. Initially, I had Deco set up as the Primary Router. However, due to some connectivity issues, I switched Deco to Access Point Mode while leaving the ISP router in Bridge Mode. This was done to see if it would resolve the problem, but the disconnections persist.

Essentially, Deco is currently in Access Point Mode, but the ISP router remains in Bridge Mode.

If this setup is incorrect or causing the issue, could you recommend the optimal configuration to avoid these disconnections? Thank you for your help!

  @TomorrowV 

My recommendation would be to keep ISP Router in its default router mode, have its WiFi turned off. Run Deco mesh in Access Point mode.

That might not fix issues you have, but it should make troubleshooting easier. Also, in Access Point mode Deco needs router, if there is no router in front of Main Deco all mesh will go offline. The configuration you describe you now have is impossible, and because it is impossible it is hard to figure what do you have and what actual configuration there is. 

Your ISP router should have reset button or reset process. I suggest you go through full ISP Router reset (if it has factory reset wiping all user settings- that is what you should do), and start anew.

After ISP Router is back online, make just two changes in it: change its default login/password (just as a security measure), turn its WiFi off. 

Then, connect Main Deco to it and make sure Deco mesh runs in Access Point mode.

  @Alexandre. Hi.
 

Just a few minutes ago, the internet went down again. Deco’s LED turned red. I unplugged the cable from the ISP router (which is in router mode) and connected it directly to my laptop – the internet worked perfectly. When I plugged the cable back into Deco, the internet immediately came back on, and the LED turned white.

This behavior is really puzzling. Could it be a compatibility issue between Deco and the ISP router, or is there something else causing the disconnection? Any further insights would be greatly appreciated.

Thank you for your time and help!

  @TomorrowV 

Next time it happens, connect laptop to second port of Main Deco, not making any other changes, and see if laptop can ping web sites and have Internet access while Main Deco LED is red.

Main Deco LED goes red in two cases: 1) it lost Internet link to the router and 2) it has Internet connection but can't ping web sites it uses for health check.

Also, because you run ISP Router in router mode, you should be able to connect laptop to a different LAN port than one Main Deco uses. Do not unplug Main Deco, connect laptop to different LAN port of router.

Just to make sure you can get Internet access through the router while Main Deco is connected to the router and has red LED light.