iPhone 5 backup to iCloud not starting
In- order to Fix the Automatic iCloud Backup not Working issue on iPhone do the following steps ;

Step:1

Delete your iCloud profile in iPhone

Step:2

Now again reconnect the icloud profile

Step:3

Now you get message sync automatically..

Well also , check this below article to troubleshoot this issue ;

https://support.apple.com/en-in/HT203271

Hope this helps...