Have checked and rechecked still only one of the cargo doors is serviced and no stairs Appear when Gateway is checked in menu
If you mean the "Parking has Jetway" option ? It's not very clear from your message, but you seem to imply that no stairs appear when the "parking has a jetway" option is enabled, as if you DO see rear stairs when that option is NOT checked. Is this the case ?
If yes, the problem is that you might have reversed the doors in the airplane customization so, they looks to be placed correctly, but they are in the wrong place, for example the Pax 1 in place of Pax 2, or Pax 2 in place of Pax 3, etc.
This because the "parking has a jetway" will disable all stairs EXCEPT for the rear one (unless the "disable passenger stairs" is checked too) and if the doors are not assigned correctly (even if they "look" right) it would explain why you don't see rear stairs only when you have a jetway.