I have a Dasan h640gw router, which I got from a company that has a fiber optic network. The problem is that I can't log in as root for a long time. I used to do it once, and now I keep showing the message that the password is incorrect, even though I haven't changed it, and I type the same one that I used to log in.
But it is still the operator's equipment - it changed access data remotely. Maybe precisely because you logged in to it as root - on the operator's servers there are logs of the work of this equipment, it checked and closed your access completely. Connect your own router and tinker there.
We don't know what the supplier is. In Vectra it is so that you can reset their modem to factory settings, and when logging into their server it will be reprogrammed to the state planned by the provider.