AT&T Internet Air

Yes, if the devices appear to be similar in regards to an available Ethernet port. Whatever an Internet25 is, I can't find a picture of this device that displays available ports.

A word of caution here regarding a Time Capsule. Apple discontinued the most recent version of the Time Capsule in mid 2018, so even if you purchased the product at that time, your Time Capsule is at least 6 years old and likely much older. Both the hard drive and power supply are in the "suspect" range at about 7-8 years.

So, you would probably not want to continue using the Time Capsule as your only backup plan if that is the way that you have been using the product up until now.

Does the AT&T device have a USB port? If yes, you might be able to connect a simple USB hard drive to AT&T device and back up using Time Machine that way.

Assuming that the AT&T device has a USB port, the next question would be whether AT&T supports Time Machine backups to a USB drive connected to the USB port on the device.

That would be a question for AT&T Support.

I can't find a picture of the back panel of the AT&T device.

Yes, if the Air has one or more LAN <--> Ethernet ports.

No, if it doesn't.