IPhone 4 No Service, but got 3G?

I recently purchased a second hand smash iPhone 4 8gb. I have since replaced the screen. The phone worked well for 2 days, however when I woke up this morning and had No Service, but did have 3G. When I shake the phone (with my fingers over the exterior buttons to ensure it's not them) I hear a light rattling inside the phone. I read online it could be part of the motherboard near the battery connector, however, the phone worked 2 days prior to the change, and I have checked the inside and at first glance can still see the blue bit I read about. Since I have been writing this, I have also lost 3G. I'm on Giffgaff, and next to me I have a Blackberry with a Giffgaff sim and that has full bars of connection. I have also tried a hard reset, and turning airplane mode off and on. Any suggestions?

Return the phone if you can. It's not just the screen that has been damaged, the antennae has also been damaged from the ratting inside the phone. The iPhone 4 uses its metal band as the antennae so if that's not damaged then its definitely that the circuitry in the iPhone is damaged. Return the phone and use your blackberry in the meantime.

Try return the phone if you think it is a phone fault but the carrier could be doing maintenance in your area hold on a few days