Direct from my fcom
”the ANP must be monitored during VNAV NPA. The only indication of the FMC tranisitioning into the “on approach” logic is when the RNP changes to 0.5 on legs page.
If RNP 0.5 is not displayed at this stage the approach should be continued, the “on approach” logic will become active at the latest when descending through 2000ft AAL.”
On another page
”the fmc transitions to on approach when the airplane is within
2nm of first approved waypoint (including approach transitions, arcs, procedure turns)
2000ft of airport elevation whatever occurs first”