Why is my iPhone 6 wireless working on b/g mode? My router is AirPort Time Capsule which supports wireless ac. My iPhone is an iPhone 6 that also supports ac mode.

I was looking at data in AirPort Utility and for my phone it was showing it is on mode 802.11 b/g which I think is old and slow but my iPad 3 was on 802.11 a/n mode.

Does that mean there is something wrong with my iPhone wireless?
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.