iPhone 4S unable to connect to TP-LINK 5 GHz Wi-Fi Router
the iPhone 4S doesn't have the 5GHz band, only the 2.4GHz
The first iPhone to support 5GHz was the iPhone 5.

I can't find an official Apple link that clearly states it, but Everymac covers all Apple models comprehensively -
http://www.everymac.com/systems/apple/iphone/specs/apple-iphone-4s-specs.html