What does the 5GHz mode warning mean?
Connectify Hotspot is using the same Wi-Fi band and channel that your wireless access point uses. Your PC acquires a connection via Wi-Fi, and if you share over the same device you use the same band and channel.
If you are trying to connect an older device that does not support 5GHz networks and you see this warning in Connectify, those devices will be unable to connect to your Connectify hotspot and will not show the hotspot in their list of networks. Most devices manufactured in the last 3 years support 5GHz and will connect fine with Connectify.
Unfortunately, Connectify does not have the ability to control between 2.4GHz and 5GHz. The channel of the Hotspot is determined by the channel of the network that the adapter is currently connected to as well as your network adapter. If you are connected to a 5GHz network, then you will get a 5GHz hotspot. The same would also be true for a 2.4GHz hotspot.
Some cards might have an option that allows you to change between 2.4GHz and 5GHz band. You should try checking in the Device Manager under the Advanced Properties of your adapter for these options. If you have those options you can try just having the 2.4Ghz enabled.