You claim the test kit is accurate, so............this implies you have calibrated the test kit and which brand is it?
How did you do the calibration?
What standards did you use then?
This does not add up.
Also, why are you doing fishless cycling with plants?
I've never once seen a fully planted tank in 20 years go through a cycle.
Even my non CO2 tanks never did.
Plants remove NH4 directly.
One of the reasons why fish do well in the planted tank to begin with.
Then you do large frequent water changes, eg, 2-3x a week for the 1st 1-2 months= no build up on any waste.
So where and how can you have a cycle with this routine?
Search the "silent cycle". Topic has come up over and over going back 20 years almost on the web now.
Then read this sticky:
Calibrating Test Kits - for non-Chemists
Do this, then see.