Why mcOS High Sierra cannot detect 5G networks?

"5G" = 5 GHz?

Check for Wi-Fi issues using your Mac - Apple Support

I have been having a similar issue. My iPhone 6s will initially connect to my 3G network, but will not connect to my 5G network until I manually go into Wi-Fi settings and connect it manually. Even when I do so, it still drops off after a period of time. That's even if I am standing right beside the Wi-Fi device.

Same identical problem here with my Macbook pro mid 2012 since upgrade to the latest version of high sierra (10.13). The problem only appears when trying to looking for the 5Ghz Airport Express network. Other 5GHZ wi-fi networks are visible....

OOPS, my bad. I was on the wrong forum. These are the steps for WiFi on a iPhone. Sorry, New to the forum. Again, my apologies.

but will not connect to my 5G network

Please make an effort to use the correct terminology, there is no, "5G network," especially confusing when you're comparing it to a cellular 3G network.

5GHz = 5 Giga Hertz WiFi frequency band

5G = 5th Generation cellular network (only in planning stages)

My solution for this problem is as follows:

STEP ONE

1. Go to SETTINGS/WIFI

2. Select the WiFi connection (5 GHZ) that you are having problems connecting to.

3. Tap the blue information button for that connection,

4. Turn on/make sure that the AUTO-JOIN button is turned on.

5. exit back to SETTINGS/WIFI

STEP TWO

1. Select the WiFi connection (3 GHZ)

2. Tap the blue information button for that connection

3. Make sure that the AUTO-JOIN button is turned OFF.

4. EXIT that screen

5. EXIT ALL THE WAY OUT.

NOTE: you should now be able to turn off WiFi then turn it back on and your WiFi will auto-select your 5 GHZ connection. If however, you at some later time need to connect to your lower connection, you can manually go into your connections and select it.

INFORMATION: This condition is actually NOT a condition caused by the upgrade other than the fact that you now have a choice to auto-join each connection. Inadvertently, this brought out the annoying little habit that your carrier has of throttling high-speed connections of customers who have a "unlimited" data plan. Unless you have your connection set for connecting at your highest speed, your provider will auto connect you at the lower setting.