Default [Q] How could I know id my IMEI was repaired?

I'm having problem with connecting my phone new phone (nexus 5 from some shop in China) to 3G. only the 2G is working.
I tried almost anything but still couldn't find the problem that cause it.
I suspect that my IMEI was repaired (I have 2 different IMEI numbers in the back of the case and inside of the back case)
I found that after repairing the IMEI with Octoplus they say that "3G network will become unavailable", How can I check that this is really the problem? And does it can be repaired?

Thank you!