Good Morning! I am assuming there is free WiFi in Gare du Nord?
It's not everywhere. I've just walked through but there is usually an area, usually near where the trains board (rather than RER or Metro), where there is a free WiFi signal.
There are WiFi hotspots, which I don't use, because your phone or laptop can be easily hacked or get a virus.
If you buy a SIM card for your phone, that would be better. About €5 in the newsstands.
Yikes! Way to get hacked! My BIL lost everything at a US airport. Use data.