Natural Cycles

I have a copper IUD in and want to get it removed as my periods have been so heavy, I also hope to start trying for my second baby in a few months. I have previously used natural cycles and got pregnant quite quickly using it but I feel it is a bit of a scam as you’re paying a monthly subscription. After looking an amazing it looks like there are free apps out there and a themometer for half the price, has anyone used any other thermometer or app and would recommend this?