This might work for some of you. I'm pretty sure most of these issues are caused by the type of Wireless Security and the Radio mode of the router. I had to rule out a lot of enviromental factors to get my problem fixed—type of case, the walls or how I was holding the phone.
I had 2 random problems with the WiFi of my iPhone 5 (6.1.3):
• "Unable to join network ..."
• If it did connect, it had a very poor ping+up/down speeds (lots of Speedtest.net results)
My home network setup:
• 2nd gen Airport Extreme (7.6.3)
• Wireless Security: WPA/WPA2 Personal
• Network extended with an Airport Express (7.6.3)
• Radio Mode: 802.11n (802.11b/g Compatible)
• Radio Channel: Auto
• Location: AEBS is in the basement and the Express is on 2nd floor.
• Router Mode: DHCP & NAT with a 365 days for the lease on each IP
• The Airport Express is setup to "Extend a wireless network"
I have 2 other iOS 6 devices and 3 other Macs which have no problem connecting to the above setup. The iPhone 5 on the other hand either couldn't connect or when it did, it would have a very poor connection. It always forced me to switch to LTE.
After many months of putting up with this problem on my phone, I finally gave up and booked an appointment at the Apple store. They wiped the phone and did the usual test with the Apple store WiFi which is freaky fast. The phone had no problems using their open network—both before or after the wipe. I was pretty disapointed after wasting 2 hours of my day.
I came home hoping the problem had gone away. Nope. "Unable to join ..." ARGHHhh... One of the solutions that people have suggested is to switch to an unsecure WiFi. That's not heppning here. So, I decided to play around with the settings and see what works and this is what FIXED my problem:
• Radio Mode: 802.11n only (5GHz)
• Wireless Security: WPA2 Personal
That's it. After the reset of both the base station and the Express, my iPhone 5 has been connected for a few hours without any problems. I've tried disconnecting and re-connecting a few times and so far so good. I hope this works for some of you.
Message was edited by: redjeko. Added iOS version.