Ok, I finally got it to work and it was my mistake. After going back and forth with Apple Support (great experience btw!) I also tried using the keys with Google and it didn't work with their system either. Well, I thought, it must be a key problem, so I contacted Yubikey support and they sent a help email with some troubleshooting steps and during this process the documentation said to "touch the disk" on the key and my key doesn't have a disk to touch, instead the keys have two small metal pieces, one on each side of the key. I thought "Surely not", but touched both sides with my thumb and index finger and that was it. So simple, yet not very intuitive. Oh well, maybe this will help someone else out with the Yubikey 5Ci key. I'll be contacting Apple support again tomorrow to let them know of the solution. Thanks again for everyone's attempts at helping!