Well, you can try to use the “Travel To” option on the toolbar to “skip” flight phases. Try to select approach, and it should jump you through the approach phase of the flight which theoretically trigger the landing assist.
I don’t know if this will work though. I haven’t used landing assist since… well… ever…
But try to use the ATC as well, when you’re on approach, you’re suppose to contact the tower to ask for clearance to land. From what I remember in past videos, the landing assist will also appears once the ATC knows you’re cleared for the approach.