I made an account just to post here. Had the same problem as everyone else, and the other solutions weren't working. Here's what did work for me: Forced my router to use different names for 2.4GHz and 5GHz network, and made the TV connect to the 2.4GHz one.
Here are the steps I did:
my router is: AT&T Arris BGW210-700. I used my mac to log into the Router's interface using a web browser. (To find your router's IP address, on your Mac go to System Preferences, Network, Advanced, TCP/IP and see where it says Router: XXX.XXX.X.XXX (except with real numbers). type those numbers into a web browser, and then you'll likely need to enter a code that's printed on the router)
In the router interface, I went to Home Network, then Wi-Fi. I saw that the router generates a 2.4GHz and 5GHz wi-fi network, and that both had the same exact SSID (name). So I then clicked "Advanced Options" and set the Network Name (SSID) to something different for the 2.4GHz network. I got a warning message about this messing up some vague optimization or whatever. I went through with it anyway. Then on the Samsung 7 TV, I went to Settings, General, Network, Open Network Settings, Wireless, and chose the newly renamed 2.4GHz network. This was the key solution. Now I can do Airplay from my Mac to the TV and it works. It works when I have the Mac connected to the 2.4GHz network OR the 5GHz network. The key was forcing the TV to for-sure connect to 2.4GHz (and not a dual-band SSID).
