Hmmm. That looks complicated. But it seems odd that I've never received one of those warnings. I have High Sierra, so the warning system should work. And occasionally I've had a dropped connection. Till now, I've been able to fix it simply by waiting awhile and then rebooting. As I said, I've done that several times for this problem, with no result.
I see that if I try the diagnostic thing, I'll need to enter my network administrator name and my password. When I got FIOS, I wrote down the very long password, and have since used it to connect devices like Alexa and an Amazon Fire. (Both of which do work for me now.)
But I only wrote down the name of the router and the password. No admin name, which suggests I didn't have one.