Iphone 6 plus wifi problems

Hello I have an att iphone 6 plus. My router is a Linksys WRT-1900ac. So my router outputs 2 signals a 2.4ghz and a 5ghz. My iphone recognizes both signals without a problem, but when connecting to them it will only connect to 2.4ghz, and will give me an error for the 5ghz. I know this isn't the router because my w8.1 pc, and macbook air work fine with both bands. Is this an IOS issue, or should I go to apple store and swap out?