R6400v2 how to turn off wifi isolation on network

> Re: R6400v2 [...]

   Firmware version?  Connected to what?

> [...] it says during the setup to turn off wifi isolation. [...]

   Is that general guidance, or a complaint about an actual problem?

> [...] the 2.4ghz network I'm using says wireless AP is on. I need to
> figure out how to turn it off for that network. [...]

   Unlikely.  Perhaps the opposite is true.  "Wireless AP" ("AP" =
Access Point) refers to a wireless-network radio.  Your "Dual Band"
router has two of them (2.4GHz and 5GHz).

   Many Internet-of-Junk devices can use only the 2.4GHz radio band.
Many mobile apps which are used to configure (and manage) these IoJ
devices require that the mobile device also be using that band, at least
during the configuration process.

   If your router uses different SSIDs for its two radios, then you
should be able to tell the mobile device to use the SSID for the 2.4GHz
radio.  The fact that the router's 5GHz radio is also active should not
matter.

   If your router uses the same SSID for both of its two radios, then
you might want to disable its 5GHz radio (temporarily) to force the
mobile device to use the router's 2.4GHz radio.

   After the IoJ device has been configured, you should be able to let
the mobile device use whichever radio band you want.  (So, if you had
disabled the 5GHz radio, then you should be able to reenable it.)

> [...] The only isolation available is on the Guest network [...]

   That should be true, unless the router's firmware is defective.