Hooking up an Airport Time Capsule

Happy to provide instructions for connecting the Mac via ethernet to the TC.. and keeping it independent of the network.. but Bob seems to have convinced you .. correctly IMHO.. that a USB drive plugged directly into the iMac is a much better choice. Cheaper, faster and much more reliable. That would certainly be my advice as well.

The AirPort Time Capsule has been designed to connect directly to a broadband modem....just as the illustration in the Setup Guide for the Time Capsule illustrates.......so your Mac can connect to the Time Capsule using either a wireless or wired Ethernet connection for Internet service.

Frankly speaking, you have purchased the wrong product. Best to return the Time Capsule for a refund and exchange it for a simple USB hard drive that you can connect to the Mac and use for backups.

You will not be able to connect your Mac to the Time Capsule USB port.....since the USB port on the Time Capsule is there only to add a hard drive for additional storage.

Whether or not it might be possible to connect your Mac to the Time Capsule using Thunderbolt / Ethernet is an open question. At best, it would be a complicated and convoluted type of setup that may or may not work.

LaPastenague has posted instructions in the past for this type of setup, but the additional security in Sierra and High Sierra has likely made this even more difficult. Hopefully, he will see this post and offer his opinion.