It could be that your IP got somehow blacklisted and router denies logins from it. Could you try setting a different IP to your computer than the one leased by the router, as instructed here? Are you able to login now?
Does the device respond the same way, when connected to another computer, smartphone?
Otherwise, not much else that can be suggested, other than try holding reset button for 12 - 20 seconds, to attempt reset to defaults once more.
Do you recall what the previous firmware version was? What custom configurations were made to the router other than default settings? Did you have any scripts, additional services running applications installed? I'd try to replicate your issue.
Best regards,