Please double-check that your device is a "Gluco+" (model number: BG5S) and not a "Smart" (model number: BG5). The most direct way to tell is by the color of the display: if the LED lights are white, it is the Gluco+; if they are blue, it is the Smart.
If the lights are white, please follow the "Gluco+" setup process in the iGluco app to connect it with your phone. You shouldn't need to press the M button for 3 seconds to enter pairing mode. The Gluco+ uses the Bluetooth Low Energy (BLE) technology, which means the device's Bluetooth is always on and discoverable in the iGluco app, instead of in the phone's Bluetooth setting.