Q: Imac 5Mhz problem

1. Go to the Apple menu
2. Hold down the option key and select "System Information"
3. Under "Network" click on "WiFi"
4. Look at the "Supported Channels:"
5. Do you have your router configured to one of the supported channels?

A possible answer to your "why":

http://www.tp-link.com/en/faq-499.html

Apple's suggestion for router configuration:

Recommended settings for Wi-Fi routers and access points - Apple Support

I had the same problem you're having with an iMac and MBA not seeing my 5GHz network. I solved it by entering my Time Capsule's (Apple router) interface and created a separate, dedicated 5GHz network SSID ( you can simply append "5GHz" to your existing network's name and use the same password as well) and then connected these two macs as well as the rest of our devices to the new 5GHz network which was now instantly visible to all machines.

Might be worth a try if all settings look good otherwise.