In the auto back up settings it says "Auto back up will start when your phone has been connected to the same wi-fi network for an hour and is charging with the screen turned off. Your phone will only back itself up once every 24 hours." There has been a lot of times where I check the last time my phone was backed up and it will say 3-4 days ago even though I fell asleep with my phone charging and connected to wi-fi. For some reason it just doesn't automatically back up.
Plus I don't get why the phone has to be charging and connected to the wi-fi with the screen off for an hour before it starts backing up? I don't mind if the phone starts backing up automatically as long as I am connected to the wi-fi, is there a way to change the requirements? I would prefer if I could choose the time when it backs up instead.
I know it has been a few days since you last posted this but has your phone got the chance to do the backup yet? As for changing the parameters in which the phone will initiate a backup, I believe this is put in place by Samsung and is not change able to ensure there is no interruption during the backup process which could lead to corrupted files. I did find some helpful tips on Samsung's website about Samsung Cloud backup that might help you. About Sync, Backup, and Restore (Samsung Cloud)
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