Given the cost of the monitor + test strips + overseas shipping, you might consider spending the money to learn the Ovulation Method (Billings, Creighton, or FAF) instead. As far as I know, it’s the only systematic form of NFP that
works reliably during lactation. In particular, it’s well able to deal with the “maybe-fertile” stage – those weeks or months, between the end of postpartum amenorrhea and the start of regular cycles, that can be very confusing with other methods.
Since they’ve achieved a high degree of effectiveness (95+%) by using the mucus symptom alone, I’m inclined to believe that a skilled OM instructor could sort out your confusion in this area. You’d need to be willing to start from scratch, though, rather than combining it with the methods and observations you’ve learned for the STM. (Part of the OM philosophy is that mixing different systems tends to lead to second-guessing and mistakes.)