Muss das WLAN beim „alten“ Router ausgestellt werden ?

> [...] Ich habe gehört, [...]

   Wo?

> [...] ich müsse das WLAN beim alten Router ausschalten - stimmt das ?

   What is your "alten Router"?  Maker?  Model number?

   If you don't want your computers/devices to use the wireless-network
stuff from your (unspecified) "alten Router", then you should disable
its wireless-network radios.

   Generally, you want exactly one router.  If your (unspecified) "alten
Router" has a modem-only ("bridge") mode, and you don't have any client
computers/devices connected to it, then you might want to configure it
that way, and use your (unspecified) "Orbi" as your one router.

   Configuring your (unspecified) "alten Router" as a modem-only should
disable its wireless-network radios.

   If your (unspecified) "alten Router" must continue to be a router,
then you might want to configure your (unspecified) "Orbi" as a wireless
access point.

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Documentation.  Get the User Manual (at least).  Read.
Look for "access point".