iphone 4s finds no mobile net


i have an iPhone 4s bought by ebay.
The Phone cant connect to the mobile net so i cant phone.

i have soldered the antenna connector on the logicboard, no results
i changed the antenna but also no results.
the baseband chip is still on the board.

is there any other ic which is important for the phone function?

sorry for my bad english i wasnt often at school 🙂