Filmware

> I tried uploading firmware [...]

   "tried" _how_, exactly?

> [...] on my Ac 1750. [...]

   "Ac 1750" is not a model number.  Look for "Model" on the product
label.

> [...] I installed the phone app [...]

   _Which_ "the phone app"?

> [...] now my phones [...]

   Not a very detailed description of those devices.

> [...] are not connecting to my router.

   A clear description of what does happen would be more useful than a
vague description of what does not happen.  As usual, showing actual
actions (commands) with their actual results (error messages, LED
indicators, ...) can be more helpful than vague descriptions or
interpretations.

   What about other devices?

   Visit http://netgear.com/support , put in your (actual) model number,
and look for Documentation.  Get the User Manual (at least).  Read.
Look for the LED descriptions and "Troubleshoot".  Pay particular
attention to something like the "WiFi On/Off button with LED" or "WiFi
LED".  Did you press it?  Further reading might not hurt, either.