can't connect, gateway ip failed...again

Hi,

We just upgraded our Xfinity modem, and now I can no longer connect to the internet wirelessly on my older Dell Inspiron. It's not a router issue-- I have four other things accessing the router just fine. It's not a service provider issue either, because again, I have access on my other devices (one of them being this newer Dell)

History:
This happened before, 5 years ago after disconnecting & reconnecting the modem & router.
I ran a connection diagnostic and got the following results:

"Gateway IP Ping failed:

The wireless router/AP for your network is either unavailable, not working, or is disconnected. You will not be able to connect to the Internet, although you might be able to connect to your local network. Contact your network administrator or wireless router/AP installer for assistance.

The gateway IP ping test determines if the wireless router/AP for the network is available and working."

And how did I get it to finally work 5 years ago, you may wonder? Well, at the time I had access to an IT person who figured out that it was a network gateway address that had gotten changed somehow, and once he fixed it, we were back in service.
Problem is, I don't know what exactly he did or how to do it again.

Thoughts, anyone? I know I'm not alone in this issue.