How to monitor data usage on all devices through Time Capsule?

Apple removed SNMP from their last generation of airports including the TC.

You need a new router if you have the tall gen5 AC wireless TC. Sadly Apple decided to go backwards.. and now of course have stopped airport production completely.

I recommend Asus models, RT-AC68U and up.. the latest is RT-AC86U which looks identical and dyslexia inducing name.. but is a great router.

Some people have gone the Ubiquiti route.. which is also an excellent choice but a steeper learning curve.

You can keep the TC while it lasts for TM backups by bridging and connecting to the main router.

Tell us if you have earlier model TC.. Gen4 etc.. as there they still had SNMP.

Also if your internet connection is slower.. <50Mbps there are some cheaper solutions around.

Unfortunately, Apple decided about 6 years ago that data monitoring on their routers was something that users did not need or want, so there is no way to do what you ask using an Apple device as the router on your network.....unless you happen to have one of the older flat or square models of the Time Capsule.

Even if you might, the next challenge will be to figure out how to install an older version of AirPort Utility on your Mac, so that you might be able to see the SNMP data generated by the Time Capsule.

Try one of the Asus models that LaPastenague recommends. You can still use the Time Capsule for backups if you set it up to operate in Bridge Mode with the Asus router.