Putting aside the ownership issue mentioned, I wonder if you could load an Arris firmware? I'm just thinking outloud. It maybe locked.
Also, what is the reason for wanting to update the firmware?
I use an old BT smart hub as an additional access point and live with the fact that it doesn't receive any updates as it's not on a BT line. It's stable regardless - I'd only do something about it if it wasn't.
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I know a bit about Wi-Fi, Telecoms, and TV as I used to do it for a living but I'm not perfect so don't beat me up... If you make things you make mistakes!