ssh trouble password as username

> [...] I was having an issue when configuring ssh. [...]

   "ssh"?  What, exactly, were you doing?  As usual, showing actual
actions (commands) with their actual results (error messages, LED
indicators, ...) can be more helpful than vague descriptions or
interpretations.

> [...] The model of my router is an AC1900 WiFi cable modem router.
> [...]

   No, it's not.  "AC1900" is a speed, not a model number.  Look at the
product label.

> [...] I was able to set up the login password for users but not for
> configuring the router because there just didn't seem to be an option.
> [...]

   You lost me.  Visit http://netgear.com/support , put in your (actual)
model number, and look for Documentation.  Get the User Manual.  Read.
Look for "Types of logins".  Which one is troubling you?

> Also it's using Http and not Https which means that it's not
> encrypting traffic, is there also a way to fix that?

   Probably not.  Why do you care?  Presumably, you're talking to your
own gizmo on your own LAN.  If someone can overlisten to that local
traffic, then you're already in big trouble.