No Internet

> [...] I have been advised [...]

   By whom?

> [...] However, it will work if I upgrade the firmware. [...]

   "upgrade the firmware" from which version (to which version)?

> [...] IS THIS CORRECT?

   I have no idea.  If I wanted to know, then I'd probably try the
update, and see if the Internet interface settings now include a VLAN
setting.  I see nothing about "VLAN" in the D6200 User Manual, but that
proves nothing, especially for firmware newer than "February 2019".  I
might even look at those settings _before_ I tried any firmware update.

   My D7000[v1] (V1.0.1.78_1.0.1) has a "Use VLANID" check box and value
field, so I would not be amazed if some other Dxxxx model also did.

> If so, how do I upgrade the firmware on my modem/router when it is not
> connected to the internet? [...]

   Download the firmware kit to some computer while it has an Internet
connection, and then attach the target router to that computer, and
follow the instructions for a manual update:

      https://kb.netgear.com/23960

> [...] (I tried with another modem/router and it doesnt work).

   You "tried" what, exactly, with what, exactly, and what, exactly
happened?

   "doesn[']t work" 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.