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 1809, Build 17763
Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.
~ Mark Twain