router – Devices keep disconnecting when DHCP lease time is longer than 2 minutes

I recently changed my home router to ASUS RT-AC86U.
I have about 12 devices connected to it via Wifi, some of them are ESP8266 mini d1 boards, which keep disconnect from the router when their DHCP lease time is expired.

Few facts and things I found from trial and error during last few days:

  • No other devices are being disconnected from the router, only the ESP8266 mini d1 boards. And it happen in all of them.
  • It did not happen with my previous router.
  • When I change the DHCP settings in the ASUS RT-AC86U to 120 seconds (2 minutes), the boards keep their connection. On 300 seconds (5 minutes) – the connection is being lost after 5 minutes.

Other steps I tried and didn’t solve the issue:

  • Because my router is connected via the WAN port to another router which is also the VDSL2 modem, I disabled the modem DHCP. Didn’t help.
  • I installed the latest official firmware, as well as the latest Asuswrt-Merlin (release & beta). Didn’t help.
  • I’ve restarted all the boards (disconnected them from the power source and rebooted them). Didn’t work.

I obviously want to set the DHCP time to more reasonable time. Any suggestions to what the issue could be?