iPhone SE and iPhone 6 only connects to 2.4GHz channel on AirPort Extreme 802.11ac @ 72 Mbps

Were all your items purchased in the same country?

Have you tried a configuration where you set a different name to 5ghz?

To start with just try a different simple name. Something like AE5ghz. You can also test it at low channel and a high channel so 36 and 161 rather than just auto.

I think the smaller iphones have poorer antennas than your much larger ipads. They also use only a single antenna whereas the ipads use 2.. but this might be mistaken.. I know my own iphone 8 tends to stick to 2.4ghz but I can force it onto 5ghz.

When you use the same name on both 2.4ghz and 5ghz the devices connect to the best signal on whichever band.. hence your iPad changing over bands when you move away into the garage because 5ghz signal there will be minimal. The phones probably stick to 2.4ghz a lot better.

Is it really a concern? If you really need better speed let me suggest you need better router. The AE were never RF powerhouses. And they do tend to slide down after the 3year over the hill mark. Most equipment does.

It is good to do a wifi survey around your house. See what kind of signal you get. This is easier on a Mac laptop for example as it is built into the OS. But there are tools for use with iOS.

Wireless diagnostics and survey can show you signal levels around your house.

You can also see a lot from the airport utility itself.

If you click on the icon and then hover your mouse pointer over each device connection to the wireless a popup menu will show the channel and signal level.. perhaps that is what you are already using. Note the signal level and you move the phone away from AE. A quick drop off will mean signal is poor on the phone as well.

Keep a log of speed, location and RSSI plus channel from the survey.

In the end it might be worth doing a full factory reset of the AE and configure it anew.

I recommend you keep both names very short, no spaces and pure alphanumerics. High marks for less than 8 characters.

Passwords same rules but can be 8-20 characters or more.

Also in the phones clear out all the old connections.. it can get overloaded and will not work properly until you clear the names and start over.

https://support.apple.com/en-hk/HT208941

What app on the laptop did you use to do the Scan you did above?

The built-in macOS app that LaPastenague used was Wi-Fi Diagnostics.

To access it:

• Hold down the Option key, and then, click on the AirPort icon on the macOS menu bar.
• Select Open Wireless Diagnostics...
• With the Wireless Diagnostics window open, click on Window > Scan on the diagnostic's menu bar.
• Then click on Scan Now to run the wireless network scan.

Great work.

The rule of thumb is 2.4ghz is stronger signal than 5ghz after you go through a few walls. But modern routers are putting more signal boost into the 5ghz stream and boosting it higher as the channels go higher.

And the sensitivity of receiver in devices is better in the low noise 5ghz environment. Plus later wave2 wireless products are getting better and better range albeit the cost of latest AX equipment is still astronomical it is going to give more range rather than speed.

Tesserax got you covered on the MacOS Wireless Diagnostics. There are lots of others with pretty maps.. but I find the clear layout of text info is just as good.

I am not sure of the best one to use on iOS.. I tried a few and gave up.

BTW If you copy and paste a line from the scan you get this far neater layout than most webpages etc will ever give you.

'asus5' <61737573 35>, bssid=1c:b7:2c:74:02:34, channel=[112, width=80], cc=AU, type=11ac, rssi=-33, rsn=[mcast=aes_ccm, ucast={ aes_ccm }, auths={ psk }, caps=0xc], wpa=(null), wep=no, ibss=no, ph=no, swap=no, hs20=no, airport=no,

'havers5' <68617665 727335>, bssid=24:a2:e1:ee:0c:c9, channel=[52, width=80], cc=US, type=11ac, rssi=-80, rsn=[mcast=aes_ccm, ucast={ aes_ccm }, auths={ psk }, caps=0x0], wpa=(null), wep=no, ibss=no, ph=no, swap=no, hs20=no, airport=yes,

How do you forget a network in MacOS and iOS? I remember seeing this somewhere but cant’t find it.

Already gave you the link for iOS.

https://support.apple.com/en-hk/HT208941

Link for Mac OS and iOS

https://setapp.com/how-to/forget-a-wifi-network-on-mac

Just type forget wireless ios or mac into google.. aint hard.

All of my gear is Country Code US except my printer has a network that the Wireless Diagnostics shows as having no US code

Printer is a client.. so it does not have a country code.. only routers.

Not sure what to do about it.

Nothing.. it is not a problem.