I think NP 17 don't have this option, only One-Shot, Daily and Weekly.
I have to do the same thing that you need, so I did this building 12 “One Shot” triggers for the specific dates.
The result of building twelve of these will be monthly executions of the Task over the course of a year
I hope this can help
In the version Qlik Nprinting june 2018 you can now use monthly and yearly schedules.
This is from the release notes pdf
Monthly and yearly schedule
Time-based triggers now allow the creation of monthly or yearly schedules.
This feature removes a gap between Qlik NPrinting 16 and 17+.