The firmware is controlled by the ISP on modems. Its part of the docsis spec.  Otherwise users could modify the firmware to whatever speed package they choose versus what they pay for.  If they’ve locked it down so you don’t have access, there isn’t anything for the user to change.

Eitherway, a modem only device doesn’t have anything to change on it. It just allows you to view connection strengths to test the connection/view the logs. 

What do you think you’d change or think will make it better?

Well, seeing, you are un-able to “TRUST”, any ISP, here. And, we are the “ONLY STATE, where COAX, is installed” (Rest of Australia, has Fibre), Oh, Government, has (Publicly), ADMITTED SCREW-UP! But unless, we pay for repair, we are stuck! Personally, I’d like to check, as to why, we are supplied with a DOCSIS 3.1 Modem, with a DOCSIS 3.0 System, does make you wonder, what else, they are hiding! Plus, no matter, what is attempted, modem, can not be accessed. Add to that, all is “Secret” ie 192.168.100.1.(/), either “took too long to respond”, or is “un-available. So is www.arris.com. The manual/data sheet, for this modem (CM8200), extremely, hard to find. Plus, seeing, ISP, monitors connection 24/7, and I’m paying for unlimited data/speed, I should be allowed to tune my system, for max efficiency.

Again, there isn’t anything to tune. there aren’t any controls. It only lets you see the connection strength you have for the users who do log in. And if they’ve locked it down, it isn’t something that the users or arris can control. 

the firmware is pushed by the ISP when they provision the devices for your service.

Sad that your local government choose cable over fibre but you’re, sadly, caught in that monopoly from your govt and the ISP. 

If you disconnect it from the coax cable and factory reset it, are you able to access the gui (without the coax connected)?