Well wireless issues are tough to troubleshoot "through the keyhole" or without any on site input. With that said, and based on the data stated so far, its possible the Wifi is load/signal balancing between the different rates and radios. It could also be a channel issue, but based on the fact Iphone could and her Ipad could not, and most new Ipads can connect at 2.4 or 5G, I think that may be relevant.
I would suggest calling Cox and talking with their Home Network department. My father called them once when I was away and they were able to help, but I don't know how things may have changed since then. Tell them you are having issues where the wireless will cut out and you think it has something to do with the wireless settings. Ask them to walk you through the different settings to see what works best for your different device. Try to do it at a time when you have the different people available in the house hold so they can all test their own devices. I would suggest these settings. Refer to page 35-42 of manual. http://www.cisco.com/web/consumer/support/userguides2/4021196_B.pdf
1. Network Mode - If you don't have any Wifi-N devices pick G only. Otherwise B/G/N Mixed. NOTE: Only available if security is AES. See Number 6
2. Radio Band - Enabled 2.4Ghz
3. Channel Width - Standard 20Mhz Channel
4. Standard Channel - Normally "Auto" is best, unless you continue to have wifi problems with the settings above, then try channel 11.
5. Wireless Security Mode - WPA2-Personal
6. Encryption - AES
7. Pre-Shared Key - keep the same, or enter what ever you want your Wifi password to be. Something easy to remember, hard to guess. Note: Any changed made here will have to be updated on EVERY device.