Hi, I'm from the company that made the module/OS that does the networking in your device.
Can you tell me your timezone so I can relate your logs to our logs? The device is staying connected just fine from what I can see, but seems like the DHCP client thinks you have a very short lease... or something.
Can you take packet captures of this traffic?
Could you also clarify which version of Merlin's build you're running and the exact router? The Merlin's website doesn't list the RT-AC66R as a supported device, only the RT-AC66U.