Is there freeware that tests a cable modem functionality?

Grandkid living a few states away calls me up this morning.
Comcast says they'll charge her if the problem is on her side.

Internet keeps going out and coming back and going out again.
Comcast says they can't see the cable modem from their side.

Cable modem was a gift from me of a Costco one (years ago).
(So that the kids could save the $10 per month modem-rental fee.)

I don't have the modem model but it's definitely NOT Wi-Fi.

She can WiFi from her Windows 10 Home laptop into the router.
The router has a gateway, which is, I think, the cable modem.
Laptop Gateway = 192.168.0.1 (as I recall)
Router Gateway = 192.168.100.1 (as I recall)

I had her ping and tracert where we can't get past the modem.

Is there any way to get a DIRECT connection from the Windows 10 laptop to
the cable modem given that there is no RJ45 port on the laptop?

In addition, do you suggest any iron-clad go/no-go test of a modem?

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My Internet is WISP so I have no experience with cable modems.