When I SSH into them it asks for a username/password. I try super/sp-admin and it doesn't work. Like I said it's odd that the very first one I added works fine. The others are up and down. I have multiple R710 and R720 that are stable so I know everything is opened on the firewall. Could it be a power issue?
If AP joined the Cloud and you try to access it then super/sp-admin will not work. You have to remove the AP from Cloud, factory reset it and then only you can SSH into it using the default credentials.
R750 needs 802.3af power at minimum for the basic functions so I dont think POE power should be an issue. Could you check how long AP shows online in portal and also check if local network connectivity and internet connection is fine or not.