802.11 protocal change needed

> I need to change my WiFi 5 (802.11ac) to 802.11b or 802.11g. [...]

   I doubt it.

> The app is with my dog's GPS tracking system and they no longer
> support 802.11ac. [...]

   General advice: You might have more success if you described
(clearly) the actual problem which you are trying to solve, rather than
asking how to implement some particular "solution" ("change my WiFi 5
(802.11ac) to 802.11b or 802.11g") which may have little or nothing to
do with the actual problem (whatever it might be).

   Visit http://netgear.com/support , put in your model number, and look
for Documentation.  Get the Product Data Sheet.  Look for "Simultaneous
Tri-Band WiFi", and notice "IEEE 802.11 b/g/n 2.4GHz".  Or, in the User
Manual, "Technical Specifications" and "Data encoding standards".

   I see no obvious reason to believe that your router would have any
particuar problem with a lame/slow device.

   There are cases where a lame app requires the mobile device where it
runs to be using the 2.4GHz band _while_ you configure the wireless
gizmo.  In a case like that, if your SSIDs are different for the
different bands, then you can point that mobile device to the SSID for
the 2.4GHz band.  If your SSIDs are the same for the different bands,
then you should be able to disable the 5GHz radios in the router,
temporarily, during that set-up operation.  Look for "Control the WiFi
Radios" in the User Manual.

   Once the mystery gizmo has been configured, you should be able to
restore the router to its normal configuration, and let your mobile
device use any radio band.  This 2.4GHz-only restriction normally
applies to the set-up mode only, not to normal operation after the gizmo
has been configured.