Your Delvfire does not directly use your mobile's data. However, the VeryFitPro app that your tracker is paired uses small amounts of data for loading maps when a GPS enabled sports mode is started, or if you've setup cloud backup for your activity data and have initiated a sync whilst not connected to WiFi.
Disable Mobile Data
You can stop the VeryFitPro app from using mobile data altogether by doing the following:
Settings > VeryFitPro > Mobile Data > Disable
Settings > Data Usage > VeryFitPro > Restrict Background Data > Enable