it might have been locked out by the ISP. I know they’ve been cracking down on the diagnostic access pages. That might have included that one. 

You really shouldn’t need to reboot it. Is there a reason you are?

My SB6120 “loses” its Comcast connection about once a day.  Once the connection is lost, the cable modem sits in a state where it does not re-establish the connection for 15-60 minutes.  Prior to writing an app, I’d recover by cycling cable modem power.  I now have my own app that monitors the connection and was able to reboot the cable modem by getting reset.htm.  The reboot immediately recovered the lost connection.  Very convenient.

It is a shame that a carrier would eliminate access to reset.htm instead of placing it behind an advanced tab that is password protected.

Note that Comcast has remotely examined the modem and determine that outbound and inbound signal is strong.  The loss of connection is unexplained but easily recoverable with a modem reboot.

In response to “shoudn’t need to reboot it”  … one of the Comcast technicians advised me to install a “lamp timer” that reboots the cable modem once a day (modem power is on all day with the exception of a half-hour power-off period), because “the cable modem works better with a daily reboot.”  That’s what we do with our modems.”

when it goes down, pull the logs and cable connections page (if you can). 

Comcast is great at saying “our line is fine” and not sending someone out to check it. 

You can also check your part of the line yourself. 

Start with removing any amplifiers, signal attenuators, or splitters from the coax.
From there check the line for kinks, damage, moisture in the line.
Check the connectors for improperly made ends, foil touching the copper coax line, loose connections, bad/old/cheap connectors, or corroded connections. Replace them if you do.
If you can, simply connect the modem right where the coax comes into the home. This prevents wiring in the home from being the issue. And some ISP’s charge if the wiring issue is in the home. So this helps prevent this.

After entering my house, I have a 2-way splitter with one-side going to the cable modem and one side going to another splitter for TVs.  I’ll check wiring again, but I’m fairly confident that my internal wiring is solid.  It may be something with the line outside of the house.

But back to the reboot… There are issues where a reboot is necessary for faster recovery.  I have found an external device that pings a remote host and cycles power when the remote host is lost.  If there is no other way to programmatically reboot the SB6120, I may need the external solution.  My cable modem is part of a network that I remotely monitor, so I need the network to heal itself for remote monitoring.