This is a problem that occurs often (but not always) on my Windows 10 laptop... I turn it on and connect to a wi-fi network; and I get the icon that means "connected, but no internet". When this happens, I go to the "Network connections" and look at the Wi-fi properties. Under TCP/IPv4, I find that it is set to "Obtain IP address automatically"; but the DNS server address has reverted to an old (and incorrect) DNS address that I had once entered manually. I click the box that says "Obtain DNS server address automatically", click OK, and then all is fine -- until this happens again.
How can I get my system to forget that old, wrong DNS address once and for all?