Walaikum Assalam,
When someone says, "Your religion is a private matter", you need to remind them that your religion shapes your life in a lot of ways, making it impossible to hide. God is not a secret. If everyone who believes in God hides their religion and belief, then how can we expect to bring those who are lost, to Him? So many people convert to various faiths daily, this would not be possible if everyone lived their lives like a non-Believer. You can only educate the world regarding the various faiths by publicly displaying your religion. When this is not harming anyone, then it shouldn’t be of concern.

As for the Hijab, you need to remind them that you're wearing it to be modest and please your Creator, not for anyone/anything else. By wearing a hijab you're wearing what you CHOOSE to wear. We can't tell someone what to wear, or what not to wear, when it's not harming anyone. You need to remind them, that just as they CHOSE to wear whatever it is that they're wearing, you CHOSE to wear the hijab.

Then you can say, are there not Churches, Mosques, Synagogues that can be seen globally. Do these buildings not represent religion? Or should they be covered, as to not display any sign of religion? What about music? Many artists mention God in their lyrics, should this be beeped out? What about fashion that incorporates religious symbolism, such as a cross? Should these no longer be sold? People who bring up this argument, that religion is a private matter don’t realize that it can’t be a private matter.

Hope this helps,
Wassalam