Admin credentials not being accepted after facoty reset

1. I performed a factory reset by holding in the reset button (twenty seconds) until the lights turned amber
2. I used admin /admin, but it was not accepted.
3. The router password reset window opens and I entered the serial number
4. Nothing happens when I click on the Continue button.

I have tried this on four different browsers, which the cache completely cleared.

O/S: Windows 10 Pro for Workstations with 64 GB Ram and 2 TB storage.