How do I set the same DNS for both Extreme and Express?

No need to manually set DNS on any of the extending devices.....that is handled automatically during the setup.

The real issue here is not DNS.....it is that iOS devices like the iPhone and iPad do not usually switch wireless access points automatically when there are multiple devices delivering a wireless signal.

The iOS devices tend to connect to the first wireless router that they see.....and if you move the iOS device near another wireless access point that has a much better signal......the iOS device will stubbornly try to hold onto the original wireless access point.

Not much that you can do about that.....nature of beast with iOS devices due to their much simpler operating systems and antenna.

If you want to move the iPhone or iPad to another area, try turning off Wi-Fi on the device, then move it to the other area near another access point, wait a minute or two, and then turn WiFi back on. That will usually force the iOS device to pick up the signal from the closer AirPort access point.

By contrast, a Mac computer will automatically switch wireless access points as you "walk" it through the house to pick up the access point with the strongest signal. That is the way that iOS should work as well, but we'll have to wait to see if Apple ever delivers on this.

Lately....maybe due to an update....my iPhone will sometimes switch to a closer access point after a few minutes. My MacBook Pro switches quickly and automatically as soon as it moves closer to another wireless access point as I walk it down the hall.