Internet keeps dropping when connected to router via ethernet cable

I have a BGW210-700 router from AT&T. The router is sitting on my desk, and it's connected to my Dell laptop via an ethernet cable. I have turned off the Wi-Fi on the laptop (so there should be no confusion about how the internet is connected). The problem is that when the laptop is connected via wire, the internet connection drops multiple times an hour, for anywhere from 0.5 sec - 3 sec at a time (in the Firefox browser, I see a "No internet connection" message pop up briefly). If I disconnect the wire and use wi-fi, everything works fine and there are no connection issues. I could do that, but if I run over wi-fi, my download speed is 10Mbps slower than wired, so I'd prefer to use the ethernet cable if possible. What can I do to fix this?

Fwiw:

1) I've tried two different ethernet cables, and both have the same problem.

2) I've tried plugging into two different outgoing ports on the back of the router, and both have the same problem.

3) I don't use a VPN.

4) This has been going on for a long time (over a year). I've just exhausted other options and am finally posting here.

5) Again, I have never seen this problem with any devices (laptop, phone, etc.) connected via wi-fi. Also, I have a DirecTV Stream device connected via an ethernet cable, and I've never seen the problem with that cable (and this is one of the two cables I tested with in (1) above).