You are encountering this Registered on network error because of
I found the information here http://www.technobezz.com/fix-samsung-galaxy-registered-network/
A) Your IMEI # Is null or have changed. I checked mines by Dialing * # 0 6 #. if it's nulled then you will see 0 said technobezz
B, Another solution they said was to update your phone - going to Settings>About>Update
C, You may also have a Unknown baseband and you don't know. When you have an unknown baseband, normally your EFS folder(Contain all the vital information about your phone ) Is corrupted . They stated the you need to backup and restore your EFS folder by using a simple tool
D, Most people found a solution by either turning radio off or on. You can achieve this by dialing *#*#4636#*#* .
You'll see Phone information, Run a quick ping then turn your radio off
This will ask you to restart your phone
If you need any clarification let me know, i'll quickly reply!