No internet through wifi AND Ethernet?

Hi, I'm running into strange internet issues - hope someone can help!

Basically, when I run internet diagnostics, it tells me the internet connectivity is working fine and it shows full bars on the wifi indicator. But whenever I try to access a webpage, it just gets stuck at the status "resolving host" and stays there. Page doesn't load.

I've tried disconnecting wifi and connecting via Ethernet/cable. No internet connectivity this way either! I've spent 2+ hours on the phone with ISP service line and we pretty much tried everything we can think of. This includes deleting the wifi network settings and adding it again, forcing DNS with 8.8.8.8 and 8.8.4.4, etc. At this point, we're quite certain it's not an ISP issue (especially since all the other laptops and devices in the house hooked on to the same router are running fine).

The weird thing is, once in a while, just after I tweak the wifi settings (DNS or TCP/IP settings), there WILL be internet connectivity again but it only lasts for about 5 seconds. So I can try to Google something and it would load the results. But by the time I try to click on a link on the results page, internet drops again and I'm stuck with the "resolving host" message in the bottom left hand corner of the browser again. So it seems like the internet connection is *mostly* dead?!

How is this possible? Any idea if this is a hardware or software issue? Has anyone run into this problem before?

(And in case anyone's wondering, yes I've searched through past threads and some people have had similar problems and I've tried those fixes with no success. Don't think I've seen anyone had issues even when connecting via Ethernet either.)

If you have any idea what could be wrong given this situation, I'd really appreciate some guidance!

Thanks a lot in advance!