Iphone blocked - blacklist problem

Hello,

I got an iphone 6s from my sister which is bought in Vietnam (she is the first owner). I tried it with a SIM from Free mobile (France operator) and one from Viettel (Vietnam operator) and everything worked well in Vietnam (with data roaming). However when I come back to France, iphone can not detect any network signal from Free SIM, I can not call or receive message but I can still access to 3G/4G. I tried to contact Free mobile's assistance and they said me that my phone is now blacklisted by IMEI. And my phone can only be used in Vietnam and they can not do more to help me. I don't know how to unlock it, Apple, can you tell me that you have an offfical support for this problem ?

You will find some information for my iphone below :

IMEI : 355*****

I'm looking forward to your response,

DO