cleaning up bad IP addresses

> We just bought a router extender [...]

   Maker?  Model number?  And then you did what?  Any useful information
at all?

> [...] We're having trouble connecting [...]

   "trouble" 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.

> [...] tech support told us we have a bunch of bad IP addresses on the
> router that are mucking things up. [...]

   Sounds like nonsense.  _Whose_ "tech support"?  Which you contacted
how?  Did you do a Web search and get the phone number of some scam
artist, or did you start at http://netgear.com/support , or what?

> [...] They want $100 to fix. [...]

   Who, exactly, are "They"?  And, if you don't know, then why would you
consider sending funds?

> [...] Any help on how to clean this up ourselves? [...]

   Not from me, unless you reveal some of the info which is missing from
your problem description, so far.

> Model: R6080|AC1000 Dual Band WiFi Router

   Firmware version?  Connected to what?  Other than the mystery "a
router extender", does it work as expected?  If the router's working,
then I might consult the folks who supplied the mystery "a router
extender".