Deco M5 WiFi disconnects with internet multiple times daily after firmware 1.9.1 update (AP Mode)

Problem Summary

Since updating to firmware 1.9.1 (Build 20250909 Rel. 37570) approximately one week ago, my Deco M5 mesh network experiences intermittent WiFi disconnections, occurring roughly 6 times per day at random intervals.

Symptoms

• WiFi devices lose internet connectivity but remain connected to the Deco network
• Wired connections directly to the main router (KPN Experia Box V10) remain stable throughout
• The problem appears to affect the main Deco unit that is connected to the router
• Disconnections are random and not triggered by any specific event

Network Configuration

Hardware

• Mesh system: 3x TP-Link Deco M5 (with 1 spare unit available)
• Main router: KPN Experia Box V10 (fiber optic)
• ONT status: PON and LAN LEDs on, Power LED off, Alarm LED off (ISP confirmed minor noise on fiber signal)
• Connection: Main Deco connected to Experia Box via new Cat5 Ethernet cable

Deco Settings

• Operation Mode: Access Point (changed from Router Mode a few months ago)
• Firmware: 1.9.1 Build 20250909 Rel. 37570
• Smart DHCP: Disabled (DHCP handled by Experia Box)
• Beamforming: Disabled
• Fast Roaming: Disabled

Additional Configuration

• IP address reservations configured in Experia Box for all Deco units
• Multiple smart home devices connected (Home Assistant, Zigbee devices, etc.)

Troubleshooting Already Performed

1. Verified wired internet remains stable during WiFi outages
2. Confirmed ONT is functioning (PON + LAN LEDs active)
3. Disabled Smart DHCP on Deco system
4. Disabled Beamforming and Fast Roaming
5. Verified Deco is in Access Point mode

Timeline

• Deco M5 system has been running for several months without issues
• Changed from Router Mode to AP Mode a few months ago (no issues after this change)
• Updated firmware to 1.9.1 approximately one week ago
• WiFi disconnection problems started/worsened after firmware update

Questions

1. Are there known issues with firmware 1.9.1 in Access Point mode?
2. Is there a recommended stable firmware version I should downgrade to (1.8.5)?
3. Could the IP reservations in my main router cause conflicts with AP mode?
4. Should I try swapping the main Deco unit with one of the satellites to isolate the issue?

System Information

• Deco Model: M5
• Hardware Version: [please check your unit]
• Firmware Version: 1.9.1 Build 20250909 Rel. 37570
• Operation Mode: Access Point
• Number of Deco Units: 3 active + 1 spare
• ISP: KPN (Netherlands, fiber optic)

Any help or suggestions would be greatly appreciated. I'm willing to provide logs or additional information if needed.