Able to connect to 5Ghz but not 2.4Ghz wireless network

So Im having trouble connecting to my netcomm router. It broadcasts in both 2.4 and 5Ghz at the same time. I have no trouble connecting to the 5Ghz but trying to connect to the 2.4 just gives me a ! over the wifi icon.
My printer seems to only pick up the 2.4 which is why I am trying to connect to the 2.4 so I can print downstairs.

Im using a late 2011 iMac.

I would also like to point out that I can connect to the 2.4 with my 6+ and other devices in the house.