Cannot access Internet intermittently

This is an example of what I experence intermittently .

On 2019/08/11  -  11:39am -> 12:14pm  -  For 35 minutes.

Tried the following:

• https://google.com Using: Firefox   "We can’t connect to the server"
• https://youtube.com Using: Firefox   "We can’t connect to the server"
• https://www.xfinity.com Using: Firefox   "We can’t connect to the server"

Then I tried EMAIL:

• EMAIL to mail.comcast.net   Using: Thunderbird   "Failed to connect to server mail.comcast.net"

So then I tried domain name vs IP address:

• https://secure.dnl-webs.com:2087   Using: Firefox   "We can’t connect to the server"
• https://162.249.126.191:2087            Using: Firefox    IT WORKED JUST FINE !!!

This is what was really confusing, I used the Windows Command Prompt for a NSlookup of the Comcast name server and it was "UnKnown":

• C:\Users\larry_000>nslookup 75.75.75.75
  DNS request timed out.
  timeout was 2 seconds.
  Server: UnKnown
  Address: 2001:558:feed::1
  DNS request timed out.
  timeout was 2 seconds.
  *** Request to UnKnown timed-out

I tried this again after the 35 minute problem ended:

• C:\Users\larry_000>nslookup 75.75.75.75
  Server: cdns01.comcast.net
  Address: 2001:558:feed::1
  Name: cdns01.comcast.net
  Address: 75.75.75.75

The Internet is obviously up and running during this intermittent problem.

But can anyone explain why sometimes domain names just don't work?  

And how can I fix it?