Network problem

So I have problems with european huawei p7. It was used when I bought it. After a while I discovered that phone's network is often gone or weak. So you can't call or go to internet. While driving the network is completely gone. First I thought that maybe the problem is with antenna cable I changed it with another P7 but the problem didn't changed. Next thing I did was changing the antenna itself, but still nothing changed so I ran out of ideas. I am thinking that maybe the problem is with sim card reader or maybe it is software problem? There is also another thing I should say. I discovered that the phone back cover has been changed and with chinese one. That means it doesn't have the NFC antenna in it. Since I don't use the NFC feature anyways I didn't do anything about it. But is it possible that the network problem might be related to this?
(PS! Sorry for my English)