My home network consists of my Internet Service Provider's router and three Decos in Access Point (AP) mode: two Decos BE85 (hardware version 1)  and one Deco BE65 (hardware version 2). The main Deco BE85 is connected directly to the ISP router via cable. The second Deco BE85 is connected to the main Deco via cable, through some network switches, while the Deco BE65 is connected to the main Deco wirelessly. All my home devices, both wired and wireless, connect through the Deco system, with the ISP router providing only Internet access and VOIP telephony.

In the first two months after installation, everything worked smoothly, with very rare Internet connection interruptions unrelated to the Deco system. In the last 36 hours, however, the connection between the main Deco and the router providing the Internet connection has become intermittent, with very frequent connection losses. I have tried restarting both the ISP router and the Deco network several times, but the problem continues to recur, rendering the local network unusable, except for quick tasks that can be performed in a few minutes.

I don't know if the problem could be due to a Deco firmware update (I have the automatic update option enabled and the firmware version is 1.20 Build 20250724 Rel. 20663 on the two Decos BE85 and 1.17 Build 20250324 Rel. 32980 on the Deco BE65). The Deco settings have remained unchanged since everything was working fine. In my initial setup the wireless network (only with the 5 GHz and 6 GHz bands) and the IoT network were active, with high-capacity network mode selected and smart DHCP, Fast Roaming, and Beamforming disabled. A little while ago, I tried disabling the 6 GHz band and enabling smart DHCP, but I didn't notice any improvement.

Apart from these random attempts, I have no idea what else I could try to get a stable connection between the ISP router and the Deco system. Any suggestions in this regard would be greatly appreciated.