You may never find out the reason.
I have had similar problems with my Verizon DSL service off and on for years, and had their techs come out on multiple occasions. No one has ever been able to determine a root cause (or at least one that they were willing to talk about).
My modem-router will show that it has an internet connection but since not a single device in the house can access the internet when these incidents occur clearly it does not. Power cycling the modem-router generally fixes it for some period of time (anywhere from hours to days) but it does recur again. Sometimes the drop outs are more frequent than others.
I know this doesn't really help you much, but you should know that you are not alone and that, regardless of the many ministrations already done in my case, no cause or solution has been found.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story