IPhone 4 - "No Service" on IOS 7.1

Hi,

After few days that my IPhone 4 upgraded to IOS7.1, I've noticed that I'll become loosing connection during calls and I've seen continuos "No Service".

Finally the IPhone is stable on "No Service".

I've tryed the sim in an other telephone and it's work.

I've tryed to make a restore of the Iphone, also in DFU Mode, and it still don't work.

The problems had started when I've use the Iphone in tethering mode, and from then in the Iphone tethering menù is inacessible (rolling whell).

I'd call a apple center and the first thing they said that the Iphone has broken and I've must to buy another.

I don't agree and I've search in the apple forum about this problem and I've seen that there are a lot of users that have the same problem.

I've contact apple support, and in a very kindly way, they sayd that there are no official problem at the moment: I've to wait for a firmware upgrade that could solve the problem or buy another IPhone.

Actually I've a IPhone that don't work... and I'm really upset

Is there any one that have the same problem?

Thanks

Alessandro