Replaced iPhone 4s Display And Battery Now No Service? Help

Hi, I recently attempted to replace the front and back cover to my iPhone 4s, lack of concentration in the beggining lead me to remove the battery without unscrewing/unplugging the connector and tearing the ribbon straight off the battery.

I was fuming but then proceeded with much concentration to change the display, taking care on everything i did and ordered a new battery for the phone.

A few days passed while i was waiting for the delivery so I could not check if the phone worked straight away, after i fitted the battery and after seeing 'searching...' for 5 minutes, it appeared with 'No Service'. Could not call or text from it or recieve either.

I've taken the phone apart 3 or 4 times now trying to work out what i may of done wrong but all i have seemed to do is round the screws after so many failed attempts. (New screws on the way).

There are a couple of times where i have had a signal, mainly either 1 bar or full bars but then i try to ring out and says call failed, then back to 'No Service'.

I have read through pretty much every question and solution on this website and still no joy.

The phone;

Is on UK Network EE

Recognises my SIM card

States the correct time and date

Connects to Wifi

Was on iOS5 when I first had the issue

Has been updated to iOS6

Has been restored to factory settings

Does have a few screws missing due to rounding off the screws (Hopefully new screws arrive soon)

Im unsure what a blue resistor looks like, let alone where to find it so im unsure if that is the issue,

Im really getting annoyed now all this bother just to get a shiny blue screen, somebody PLEEEASE help before I launch it at a brick wall!

Thank you

Dave