NAV / ILS Training Aircraft and Advice

I’m having trouble setting flight plans on FMS. Sometimes the Nav/Appr/ILS works fine and at the same airport on other occasions they won’t work, this is via the world flight plan and manual plan?

I want to use either the CJ4 - TBM930 - KingAir and was wondering which aircraft is best for learning in? Do any of these aircraft have any known FMS/Nav issues?

Are there any addons (Xbox) to help with this or detailed videos to show exactly what steps need to be taken?

Thanks