AirPort Extreme "Device not found" but I'm connected to it's WiFi

recently it started not showing on the Airport Utility either on my iPhone or on my MacBook Pro.

That might be a clue that "recently", you may have updated the operating system on the iPhone and MacBook Pro. Then again, it might not. Can you provide a bit more information on this?

I don't understand how can it say "device not found" when I'm actually connected to the AirPort Extreme WiFi.

There are countless other users who do not understand why this might be as well, as we see this issue all the time. It is usually caused by an "automatic" IPv6 setting on the AirPort that is not doing things "automatically" for some unknown reason, likely a "bug". An AirPort that is getting along in age can exhibit this behavior as well.

Could you tell us what model of AirPort Extreme that you have? If the AirPort is "flat" and looks like a small white pizza box, look on the bottom of the AirPort for the model number, usually embedded in the foam pad base. It starts with an "A" followed by 4 numbers. Note.....we do not need a serial number, just the model number.

If the AirPort is tall shaped, and looks like a tower, just tell us that you have the tower model.

So that we might better understand how the AirPort is configured, could we get the make and model number of the device that you call your "modem"?

I'm running Mac OS Big Sur 11.6 on a MacBook Pro (Retina, 15-inch, Mid 2015) and an iPhone on software version 15.3

So, both your Mac and the iPhone have had recent changes or "updates" in the last week or so. That might be a clue here....and it might not. We will see.

I have the tower model

Thanks, that's all we need as there is only one "tower" version of the AirPort Extreme.

Yes, it's a Huawei EchoLife HG8145V5

That is actually a modem/router or gateway.....more than a simple modem. That would mean that both the AirPort Extreme should be configured to operate in what is known as "Bridge Mode". More details on that later if needed.

I do agree it's getting along in age

Apple sold the "tower" AirPort Extreme from mid-year 2013 to early-year 2018, so your AirPort might be as little as about 4 years old and as much as 9 depending on when it was placed into service. The average useful life of a router for home use is about 5-6 years before they begin to start acting up or sliding downhill. Mine went about 6 years before it became unstable.

Although you have probably turned the AirPort on and off a few times, you probably have not have performed a complete network power cycle, so that would be a good first troubleshooting step to perform. When you have some time.......

Power off every device on your network......modem, AirPort Extreme, Mac(s), Phones, accessories, etc.....in any order that you want

Wait a minute

Power up the modem and let it run for 2-3 minutes by itself

Power up the AirPort Extreme and let it run for a full minute

Keep powering up other devices one at a time about a minute apart until the entire network is back up

Check to see if the AirPort Extreme will now appear in AirPort Utility on the Mac and/or iPhone

Report on your results