R6400v2

Make sure your device is connected to the NG wifi router. 

Uninstall the NH app and reboot the device and re-install. 

Try the app on a different device to compare. 

Netgear has set up a community forum specifically for the Nighthawk App product. Most of the people who watch that forum are more likely to have experience with Nighthawk App. Might be more likely to find someone who has a solution if the question is posted there:
https://community.netgear.com/t5/Nighthawk-App/bd-p/en-home-networking-apps

Please use this link to the main forum product list to review and choose where to make your posts. 
https://community.netgear.com/t5/NETGEAR-Forum/ct-p/en-netgear
Thank you

Any progress on this? 

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CBO2 wrote:

Router up and connect via internet and wifi, but I cannot access my knighthawk via iphone.  No problem until I installed new spectrum modem yesterday all other app working.  Have tried to put in passwork but does not accept and times out.  Even reinstalled knighthawk app still no way to see what is on via the iphone on knighthawk.  Go online via laptop everything up and working.  Thx

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No I just removed the Knighhawk app as I got tired of going back and forth, doing everything that was suggested.  I think it started after the upgrade so not worth the time. I did like seeing what was going on with the app but like I said not worth the time to troubleshoot.  Thx