Why is my iPhone 6 wireless working on b/g mode?
Most routers/AP will step down to b/g when there is a device on the network that does not support N or AC. Often this is a printer, or Roku or something you have on the network. Examine all the clients and devices to determine which are causing the router to step down, and either reconfigure, upgrade or eliminate them. Also, there may be an outside device on your wireless network causing similar issues.

Alternatively, you can configure your Airport to work in N mode only. Open the Airport Utility, select Wi-fi Settings, Radio Mode and change the mode for 2.4GHz. This is also a good way to see what devices might be incompatible with the faster speeds your router supports.