mobile

Hello!:)
This is the third phone S820 I order. The other two phones had no problems but this one came losing coverage. I read about that problem in this forum and I saw I have to change modems.
On the working phones software was v1 1347 and modem v51. On the non working phone is 2347 and v50 modem.
So I started changing - I tried v52, v12, v51, v 102 modems - some were better, some not but problem persisted - network signal changes a lot especially when I enter an elevator or bathroom and sometimes even drops down to 0 and I have no network signal.
After that I tried flashing roms - I flashed 1347 version from needrom (with a lot of chinese apps... but nevermind) and put v51 modem hoping it will be ok - No, still losing coverage and drops down completely in elevators etc ..
I tried vibe 2.0 - same..

I wonder if it is hardware problem and if you can guide me fixing it? Is it possible to be the antenna? Where it is located? I saw a video that had a wire in the left of the phone but I think this is for Wifi and I think the antenna for network signal is another?
I heard that you can put a little paper on the antenna for the signal and it will work better ?

Please give me some advice, I'm desperate. THank you!:)