@Tompabomp 

I have cable (coaxial) Internet and when similar thing happened, after extensive troubleshooting ISP Tech Support have found the issue to be cable with cracked insulation outside of my house. 

The primary symptom was drop of Internet connectivity at random times day and night, but always for the same duration of 2-3 minutes.

What type of Internet connection do you have: DSL, fiber, cable?

  @Alexandre. I have fiber.

Shouldnt I have the the same problem when I have my PC directly to the fiber port if that whas the case?

  @Tompabomp 

If problem happens at random time and of short duration, it is possible you would not notice it on a PC connected to ISP modem. Unless you spend a lot of time in front of it and constantly run video streaming.

I haven't made any changes to my ISP modem, so all four its Ethernet ports are operational. I wrote simple script for Windows PC which pings Internet host and writes to log only when ping fails. I connected my desktop PC directly to ISP modem Ethernet port, leaving Main Deco connected to another ISP modem Ethernet port.

I left PC running for 24 hours. The data I've got from that test excluded Deco mesh I suspected initially.

If you can do that type of test, I can share script with you. It will prove where issue is: with Deco mesh or with ISP.

You can also check Main Deco logs for losing Internet connectivity, but these will be different log lines from those you provided. I suggest you do the following: on Main Deco, change logs to DEBUG, clear all logs. Immediately after, unplug Ethernet cable from Main Deco, wait till its LED goes red, plug cable back and wait till Main Deco LED is OK (green or white depending on Deco model).

Then, save logs and review them - look for lines with key words "connection", "CONN", "OFFLINE", "ONLINE". You'll figure what logs lines alert for loss of connectivity and when it comes back.

I don't know about X60, but my Deco M9 does not keep logs for over few hours before it starts overwriting them. That could not give me long enough time frame to understand the scope of the problem. I ran script on PC for 24 hours and got interesting info from it: random times of outage, always almost same duration. That excluded ISP maintenance and very much points at something restarting or rebooting, taking same time every time to do that.

I am not saying what you have is definitely an ISP fault, but if you changed nothing in Deco mesh and haven't updated its firmware just before outages started, it is worth to put effort to confirm this is (or is not) an ISP issue.