If you are using an iPhone 7 upwards, you will no longer require an NFC Reader to connect your sensor to your phone. The connection will be via NFC contained within the phone.
Please ensure that you have updated to the latest version of the OvuSense App. Once you have done this, visit the user guide contained within the app for instructions on how to add your Sensor (Section 1.5.1) and to connect your Sensor to download your data (Section 2.4.1).