monitoring device activity on wifi

No.. sorry

Your profile is showing an AC version Extreme. If you had the older N wireless version the answer would be yes. Apple removed SNMP. It is used to monitor traffic. Nobody needs to monitor anything; Apple decided on your behalf.

https://en.wikipedia.org/wiki/Simple_Network_Management_Protocol

Does the modem you use also have router mode? If so it could well have SNMP.. even very cheap routers have it. You can use a standard monitoring tool like PeakHour on the Mac to track usage.

I recommend replacing the Airport Extreme. You can still use it for Wireless duties but there are far superior routers on the market now that will do what you need.

If you want to do it on the cheap.. you can buy a second hand router. Not only will it monitor your usage it can control everyone and provide quotas even.

Look at the list of routers that are supported on third party firmware Gargoyle. Just google as I am not allow to point to the URL.

20Mbps is not difficult .. a very low spec router will handle it. One I found that works well is WD mynet N600 or N750. But there are lots of TP-Link, Netgear etc. Wireless might be fairly poor so using the airport still for wifi would be helpful. Usage is still controlled by the router.

Not many support AC wireless routers support Gargoyle but these are about the best. WRT1200AC or WRT1900ACS or any model actually in that series from Linksys.

Best routers with built in controls. RT-AC68U is the cheapest of the series.. and tons of them available cheap second hand.

Asus higher end models. e.g. RT-AC86U or AC88U or AX88U. They do have DSL models as well.. if you were prepared to replace the current modem but check your local situation if they work. For example in Australia the DSL-AC68U is problematic. You are better off keeping whatever DSL modem you have now in bridge. The better end of the range will be useful long term when/if you get higher speed broadband.

You could also replace it on the wireless side.. if you use the DSL modem as a router or stick with the Airport, but turn off the wireless and use a decent wireless AP unit like EAP225 or EAP245. These give you extensive controls although not as good as at router level.

Lots of way to skin the cat.

Edit.. Bob got in first and covered most of it .. morning Bob!!

Unfortunately, there is nothing in AirPort Utility or any other Apple software that would tell you which devices are using what bandwidth at any given time.

Most apps that do measure bandwidth use by device make use of a reporting system called SNMP. Unfortunately, Apple decided that users no longer needed useful things like logging and bandwidth information, so they dropped SNMP off of their routers starting in 2013. I doubt that you are going to get much information with an Apple router, even with professional applications.

I have noticed that the upper end Asus routers allow the owner to monitor network bandwidth use by device, so you might want to take a look at a new router if bandwidth monitoring is important to you. As you know, the most recent AirPorts were designed in the 2012-2013 timeframe, so they are already more than "long in the tooth".