This problem can be fixed by turning off the combined encryption of wpa/wpa2 and using only WPA2 for both the 2.5 and the 5 Ghz data streams. Once you log into the router and change the settings, your Mac should be able to connect to both without problem but be sure it is only set to WPA2 for security and not the combined security method.
This worked! amazing!! There's an annoying little orange light showing on the front of my Homehub 4, but I can finally put away the ethernet cable that I've been tripping over for the past five months. thank you thank you thank you.
Registered just to say thanks to Karen09. Switching to WPA2 only fixed my problem without having to unmerge the channels. My Macbook and phone connect flawlessly now.
Good answer however this thread is over three years old and last posted on two years ago so you may not get a reply from the OP but your answer may help somebody else.
This worked perfectly for my MacBook Air 2015 running macOS Mojave. Thank you!