No 5G wi-fi on MacBook Pro after El Capitan upgrade

After recently upgrading to El Capitan my MacBook Pro 13", 2012?, will only connect to 2.4 wi-fi. The 5G wi-fi will show in the list of "available" networks but when I try to connect I get, "Network in unavailable try moving closer to cable modem". Any help, please. I have contacted the provider and have an Apple TV that has connected to the 5G.

Mark