No service on my iPhone In top left corner it is written that no service in settings the carrier name is wrong and when I select the carrier manually it says the network does not exist 
P.s. I ported my sim to a new carrier few months ago and used it normally but now suddenly in the settings it shows my old carrier
Ask your carrier for a new SIM.
The firmware on them changes over the years, so yours may well be long out of date now. 

[Knowledge source, recent similar issues &amp; similar cure]
My phone would drop the carrier network every few days, with a message that 'if this happens frequently, talk to your carrier.'
I did, they sent a new SIM, it's not misbehaved since. The 'old firmware' explanation was theirs, over the phone, so I have no documentary evidence.