Hello,
you have to use the Bb-fingerings.
N/A means, this note is not playable on the horn. This is because of the harmonic series. These very deep notes are between the first and the second natural note. The interval is an octave!)
Yes, you want to use the Bb fingerings for your Bb horn.
The n/a's technically do not exist on your horn, but there are trick fingerings you can use to with that stopping valve that will allow you to get them. Of course you may not see those notes written for a long time yet, and when you do they will will probably be old notation which will mean they are actually up an octave.