Imac not connecting to my 5 g

I have a asus RT68U with 2 bands. My iMac isn't connected to the 5G. I don't know how to force it to. My other iMac connected to it fine.
DO i have to set this on the iMac or the router itself?

If others are connecting to the 5G just fine then the router most likely not the problem.

More information would help. What is the year and model of your iMac? Also how close is it to the router? 5G is a short range signal and if the iMac is too far away or the Airport card in the iMac is weak, it won't be able to connect. You might also check the settings on your iMac. Go to System Pref -> Network -> Advanced -> TCP/IP -> make sure IPv6 is set to automatically. You may need to click the little gold lock and input your username and password to access the page.

Lisa

It's a 2017 5k . My 2014 iMac connected to it fine. same router and all.
I checked and set IPV6 automatically is set.

Edit: I checked my router settings and IPV6 was disabled. enabled it and now 5g!

Thanks for the tip

Q1. How far is the 2017 from the router?
Distance is a consideration. 5G is a short range wireless signal. You could try to force it by turning off IPv4 but I have a feeling that would result in no connection.

Q2. How are you determining if it is connecting to the IPv4 or IPv6?
I can tell on my wireless as I have given each one a different SSID. When I am close to an access point I will pick up the 5G but as I move away with my MB Air, it will switch to the 2.4G.

If distance and settings are not the issue I would give Apple a call as being a 2017, it is under applecare and it could be an aircard issue.

Lisa