Before cutting the opening behind the toilet
for the access panel make sure there is nothing that could be damaged such as water pipes and or electrical wires.
Carefully drill a 2.5″ opening in the center of the taped access panel area so you can see inside the wall.
Proceed only once you’re sure nothing is in the way and is safe to do so.
Using a 2.5″ Forstner wood drill bit, drill through the header up in the attic directly behind the toilet and drop down the 1.5″ ABS vent pipe to the access panel opening.