Which program to use to monitor connection stability?

I get disconected shortly but frequently - but this is lan, not wi-fi! Used to work fine. Now I need to determine is it windows fault or ISP. Google gave me only complicated Linux programs.
I just want it to track and record how many times I am offline and for how long?

Try pinging a server like cloudflare's 1.1.1.1 IP using command prompt or what equivalent your OS has: www.ivpn.net/knowledgebase/troubleshooting/how-do-i-use-the-ping-command/
It'll continually update at intervals to show if you are connecting and you can read through it to see if you are getting nothing at times. You can also check how you are connecting to Lichess here and watch the numbers update in real time:
lichess.org/lag

A possible few solutions:
-Unplug your modem for 5 minutes to reset it.
-Flush the DNS/reset your net connection: support.pearson.com/getsupport/s/article/Reset-an-Internet-Connection-Flush-DNS
-Restart your PC.
-If you think it is your ISP, tell them as they may not even know. They may have to come over repair something as was the case with our's at a few points.

(Bonus note: Google search has been pretty bad for finding stuff for a long time now because at some point they started favoring their own products/services/places-that-serve-their-ads and filtering out others. I highly recommend switching to DuckDuckGo or Ecosia to get some more thorough results away from their bubble and they are more private than Google's data tracking dangers.)

Thanks.
Btw. I've found some online ping tracker, which was fake - showing pings and spikes even when I was offline :)

You're welcome!

Wonder' why they'd even make one like that, lol! Maybe as a prank or trying to sell something to supposedly improve stability or something?