D6200 won’t connect to NBN (HFC)

> Model: D6200|Dual Band 11ac ADSL Modem Router

   Firmware version?  Connected to what?

> D6200 won't connect to NBN (HFC)

   "won't" is not a useful problem description.  It does not say
what you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

> [...] the wifi light is amber [...]

   _Which_ "light"?  What does _Netgear_ call it?  Visit
http://netgear.com/support , put in your model number, and look for
Documentation.  Get the User Manual (at least).  Read.  Look for the LED
descriptions.

> [...] our NBN box. [...]

   All that non-description says is that you're in Australia; it conveys
no useful information about the device itself.  Maker?  Model number?
Actual ISP?

   "(HFC)" suggests that there might be a coaxial cable involved?

> [...] Have read that settings need to be changed [...]

   "read" _where_?  _Which_ "settings"?  "changed" _how_?

> [...] am not sure how to do this without a connection. [...]

   Read further in that User Manual cited above?  You don't need an
Internet connection to talk to your own router.

> [...] (NBN is active and shows it's all working on the modem)

   Your conclusion is mildly interesting, but the actual evidence on
which you're basing it would be much more interesting.  Along with an
actual description of that "the modem".

   Was your D6200 previously used in some other (DSL?) environment?
Have you tried doing a settings reset, and then configuring it afresh?