Click the Wireless Network Options button (I don't see this option on the Expresses, but do see it on the Extreme)
Sorry, I run my remote wireless access points with ethernet tied back into the main hub or router and forgot that you had them set to "join" or "extend"
You'll need to temporarily change the Wireless Mode to "Create a wireless network" and then the Wireless Options button will appear. Make your changes and then change the Wireless Mode back to "Extend" or "Join" according to your needs.
When the AX "joins" it does not provide any additional wireless coverage in the area where it is located.
Uncheck the box next to Interference Robustness if it is checked (I don't see this option anywhere on any device {ver. 7.5.2}
Newer devices sold in the last 6-7 months do not have this setting.
but I do see "Use Wide Channels"
You should definitely have a check mark next to this option
Does checking the 5GHz box here mean it will not run in 2.4GHz mode if selected?
This can get somewhat complicated. You have a
simultaneous dual band AirPort Extreme. That means that it is broadcasting both a 2.4 GHz band or network and also a 5 GHz network....but both bands use the same wireless network name. That's the best setting for most users because the computer or device will automatically choose the band with the best available signal.
If you have a newer computer and you have a good signal, it will probably connect to the 5 GHz band. An older computer will not be able to connect to this band, so it will connect to the 2.4 GHz band.
5 GHz signals are not as strong as 2.4 GHz, so if you are outside on the patio, your newer computer will likely connect to the 2.4 GHz band because the signal is stronger at that location.
It's an open question whether your Airport Express ( which is only a single band device) is extending the 5 GHz band or the 2.4 GHz band. It cannot do both at the same time since it is a single band device.
Move your laptop near one of the AirPort Express devices and hold down the option key on your Mac while you click on the fan shaped AirPort icon at the top of the screen and you will be able to see which band your computer is connected to at that location.