What else do i need to do? Iphone 5 'invalid sim' problem?

Done these:
1.Update your iPhone to the latest version of iOS (7.1.1).
2.Toggle Airplane mode On and Off. (doesn't work)
3. Try turning iPhone off and then on again. (works for a minute)

i recently replaced my sim to a nano one. And asked them to turn the gprs off cause it spiked my bill

I agree with Thomas post.
If you are interested to unlock your iphone 5 through online unlocking service means try http://www.wickedunlock.com/apple-iphone-5/rs5wp9/ Here you can unlock your iphone by Remote unlocking method.It's one of the best and safe way to unlock your iphone without jailbreaking. After that you will get relief from invalid sim problem.

Get your iPhone carrier to unlock the iPhone or take it to someone who can

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