Wifi on iMac slower than other devices

The download speed on my iMac is typically less than half the speed of other devices connected to the same wifi network and in the same room. Sometimes is slows down so much it can't load a web page. What could be causing this?

Example -

iMac

18.6 Mbps down / 12 Mbps up

iPhone 6s

52.4 Mbps down / 13 Mbps up

Both are connected to the same BT hub on the 2.4GHz band. iMac/iphone are located in upstairs bedroom, router is downstairs.

iMac is running Sierra 10.12.6

I've already tried separate SSIDs for my 2.4 and 5GHz bands. The iMac refuses to connect to the 5GHz band. for some reason. I'm sure it used to switch between 2.4 and 5 but now it seems to only like 2.4.