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.
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