Bridge Mode?

> Does this modem come from the factory in Bridge mode? [...]

   It's a modem, not a modem+router.  But why trust me?  Visit
http://netgear.com/support , put in your model number, and look for
Documentation.  Get the User Manual.  Read.

> [...] I'm having problums port forwarding. I have set up the netgear
> router (AC1200) with no luck on any ports.

   A modem-only device like a CMxxx should not affect port forwarding;
that's a router function.  What, exactly, is your "the netgear router
(AC1200)"? "AC1200" is a speed, not a model.  Look at the product
label.  Firmware version?

   What kind of problems?  "problems" or "no luck" 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 with
their actual results (error messages, LED indicators, ...) can be more
helpful than vague descriptions or interpretations.

   Is there some actual problem which you are trying to solve?