iPhone5 WiFi issue

TLDR: iPhone5 has poor wifi coverage. Signal is fine within 3-feet of router, but drops about 10% with every step taken further from it.

Question: What would you do at this point to resolve?

Diagnostics Method:

I have a total for four iPhone5 devices in my household. This problem only applies to one of these devices. I also have 4 different routers (Buffalo, Meraki, D-Link, TP-Link) available which I've verified that the issue persists with all of the routers on different channels and various 802.11 protocols (a/b/g/n). The phone in question was running 6.1.3 when the issue began about 2 weeks ago.

Solutions Tried:

1. Reboot the router
2. Tried "forget this network" and add it back
3. Reset the phone
4. Airplane mode (on and off)
5. Double hard reset
6. Update to 6.1.4 and not restore from backup + update carrier settings
7. Switch from AES encryption to TKIP
8. Tried WPA2-Enterprise network (since I had one setup)
9. Tried an open WiFi network
10. Called Appled and they did a remote diagnostics (came up clean)

The problem is apparent just by holding the phones side-by-side. My personal phone shows a full signal, the other shows 2-bars or less and constantly fluctuating.

My next step is to have the phone swapped in store. This will take a bit of time to get done which is why I've posted this.