Unlike a router, where you can download a file and hit "update," cable modems like the MB8611 are strictly controlled by the Internet Service Provider (ISP).
You cannot manually download or upload firmware to this modem. ISP Control motorola mb8611 firmware
The culprit wasn't the hardware, but a between version 8611-19.2.20 (stable) and the newer 8611-21.3.7 . Unlike a router, where you can download a