Need to find Network Security Key on cable modem CM 500V to install new mesh network

> Need to find Network Security Key on cable modem CM 500V to install
> new mesh network

   That might be difficult.  Who says that a CMxxx _modem_ _has_ any
kind of "Security Key"?

> I cannot connect my computer [...]

   Not a very detailed description of that device.

   "cannot" 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.

> [...] to my new eero mesh network [...]

   Not a very detailed description of that device (those devices?).

> [...] without providing the network security key. [...]

   _Whose_ "network security key"?

   Are you following any set of instructions?

   Visit http://netgear.com/support , put in your (modem) model number,
and look for Documentation.  Get the User Manual (at least).  Read.
Look for anything like a "network security key"?

   If you're trying to make a wireless connection between your
(unspecified) "my computer" and your (unspecified) "my new eero mesh
network" router, then you'd seem to have an argument between two
non-Netgear devices, and, if so, then you're looking for advice in a
sub-optimal place.

   Presumably, documentation exists someplace for your (unspecified) "my
new eero mesh network" equipment, too.

> [...] I can't find it when connecting directly to the modem using the
> IP address

   That makes much less sense than you might think.  For the
non-psychics in your audience who can't see any of this, physically,
what's "connected" to what, exactly?  (Hint: If a device has different
types of Ethernet ports, then "connected to device" is not enough
detail.)  And then what did you do?  _Whose_ "the IP address"?  What was
that "the IP address", and what, exactly, did you do with it?