All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Covering the face is obligatory for the Muslim woman according to the preponderant opinion of the Jurists. We have pointed out that in Fataawa 83033 and 83702.
Since you believe that Niqaab is obligatory, then you have to adhere to it for it is obligatory for you in any country you live in. Hence, it is not permissible for you to migrate to a country that bans that (wearing Niqaab) if there is no necessity or dire need to do so. For more benefit, please refer to Fatwa 193809.
Also, you should know that living in any non-Muslim country cannot be free from dangers against one's religion in general. For details, kindly refer to Fatwa 86405.
Based on that, we advise you to try to convince your fiancé to leave that country and migrate to a Muslim country to work in or marry you and leave you in the country that you are currently in and come to you from time to time until he becomes able to live in a Muslim country.
This is if he really wants to marry you. Thus, if he is convinced, then praise be to Allaah. If he refuses and insists on taking you to that country after marriage, then do not agree to that for it may cause one to be tempted in his/her religion and to commit religious violations. So, ask Allaah The Almighty to facilitate for you what is in your best interest.
Allaah Knows best.