Cannot connect to phone service after downgrading. Help me!

I flashed my phone to android 5.0 with a custom rom. Then after using for about a month or 2, I think its not good and flashed back to stock rom with odin. I chose android 4.1.2. Then i cannot connect to phone service. I tried to upgrade it to android 4.3 with kies stock rom and it can connect to phone service after that. now i downgrade it again back to android 4.1.2 and i cannot connect to phone service. help me!