Looking at the pictures, https://store.arduino.cc/usa/arduino-mkr-nb-1500, the antenna connector seems like it would be very close to the edge of any shield that was attached to this board. If a shield was installed, it might block the antenna connector and prevent the attachment of an antenna.
It might be better to to have the antenna connector on the edge of the board and pointing away from the board itself. You wouldn't have to worry about any shield interference then. You could use an antenna with a right-angle connector in order to make the antenna stand at a right-angle to the board itself.