How can I Install guide for AirPort Extreme?

my main question is whether I can use wireless Wi-Fi feature to provide a spare iPhone as a continuous internet provider to the AirPort device wirelessly

No, the AirPort Extreme requires a wired Ethernet cable connection at its WAN "O" port from a cable modem, a DSL modem, or another modem/router or gateway for Internet access.

then use the device as a Wi-Fi router to our two Mac’s and our Wi-Fi printer (wireless)

No, same answer as above.

It would be really goofy, but if you happen to have an AirPort Express model A 1264 or model A 1392, you might be able to configure the AirPort Express to receive or "join" the wireless signal provided by the iPhone hotspot and then connect an Ethernet cable from the AirPort Express to the WAN port on the AirPort Extreme and set up the AirPort Extreme as a Wi-Fi router.

The AirPort Express does have a special feature that allows it to receive a network Wi-Fi signal and then provide a wired network signal at the Ethernet port.

The AirPort Extreme is a DSL or Cable router.

You need a cellular router. AT&T has them. Please contact AT&T Support to go over your requirements.

https://www.att.com/buy/connected-devices-and-more/

Good luck.

Which one? There are 5 different versions of the AirPort Extreme.

[Edited by Moderator]