Wifi Router Needs to be reset daily

Hello TechSupport! I am hoping that the internet-at-large, can help me troubleshoot my problem...as I'm not really getting anywhere with Cox.

Here's the story: I am having troubles with my internet connection, and needing to reset the wireless router (Apple Airport Extreme) every day to get online.

History: About two months ago, I started having issues as mentioned above. Initially, it would require a reset about once a week, than it gradually became more frequent. To a point at which I had to reset once a day now.

Equipment: I have a late 2012 iMac, with Cox internet in Phoenix. I have a Cox supplied Cisco Phone/Internet modem hybrid. I was also using a Apple Airport Extreme (5th Gen/Flat model).

When the internet is "not working" - I can tell by using the Airport Utility built into my phone, or my iMac, and see the internet connection is yellow - and "not connected". At which point, all I have to do, is unplug my Airport extreme, and reconnect, and I was back online. This lead me to believe my Router was going bad (at that point was about 4 years old).

What I've done so far: 1) I purchased a brand new Apple Airport Extreme (latest generation) and set that up the same day. Within 24 hours, my internet was out again, and the trick of "restarting the router" worked again. Within 30 seconds, I'm back online. 2) - Thinking maybe the modem was bad, I called Cox, and they validated that something may be wrong with the modem. I had an older phone/internet modem lying around, so I had them swap out the active modem to the older one. 3) - After I did this, Cox was able to see the modem again and everything looked good. In fact, I ran this way for a month, and never once had an issue. However, the older modem was somewhat limited in it's speed, only getting an average of about 25-30 mbit per sec or so. 4) - Just a few days ago, I went to a Cox store, and they replaced my other modem, that was the newer DOCSIS 3.0 version (the one that was supposedly not working) with one of the same model. 5) - They activated immediately, and I went home that day, replacing the modem, and once finally online, verified my faster speeds, and all was good.

Or so I thought.

24 hours later, and since last week, I've had to once again, reset my router to get online.

This is a new modem, a new router, etc - so it's leading me now to believe, maybe something with software, but I don't even know where to begin. I may see if Cox has another modem (different model, etc) to see if that helps, but if anyone has any other ideas I could try, would love to hear it!

Thanks