How to update firmware on my cable modem/router?

> Wierd?

   No.  Cable-TV ISPs generally want to control the firmware on cable
modems which are connected to their networks, so you may not be able to
update the firmware on your cable (TV) modem.

> My question is - what is this device while I can't find it on
> Netgear's website

   Netgear makes various devices which are sold through ISPs, not retail
channels.  The actual Netgear customer (your ISP) provides support to
their customer (you).

> and why did ISP modified it's firmware, so I can't update it?

   More "specified" than "modified", as explained above.  You can ask
your ISP if there's an update available, but there may be little more
which you can do.