I know i used to have fibre for 4 years in a different city. I found another possible issue in the router logs:
14.07.2025 22:08:07 [Steering] (Mac address of my device) RadioId:0, Rssi:-42, targetBand:0, steerReason:2
The log is full of these messages. I think the firmware is causing steering logging even when I turned off band steering and only use 2.4ghz and this is causing DHCP instability.
This person had a similar issue:
https://www.geekzone.co.nz/forums.asp?forumid=39&topicid=315188