Network signal blinking after changing sim card.. help!!

Hi. My brother's note is not rooted and he didn't do anything in it. Today he changed his sim card and when he started up his phone, he cant make any calls or sms. Network signal changes every second from having network to blocked icon. Both sim cards belong to the same network company. He changed back to his old sim card and showed the same. Warranty is out and im ready to fix it if it needs any firmware changes or rooting but he doesnt want to do any of that. I will do it if there is no other solutions.
Thanks

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