ANSWER: No. Please see the link below for details about KRACK.
As you can see, KRACK only applies to WiFi devices. All Motorola cable modems with WiFi are access points. KRACK doesn't target access points. KRACK targets smartphones, computers, and other devices you connect to the wireless network. This means that you should try to make sure those devices are not vulnerable to KRACK. In some cases this is as easy as making sure that your firmware for those devices is up to date.