Reading/writing long values to BLE device via Qt

I'm writing a Qt 5.5.0 app that communicates with a Bluetooth low-energy peripheral LinkitONE. The peripheral is pre-programmed in such a way that to read or change a variable first I have to write the variable name into a specific characteristic and then either read or write the data from another characteristic.

I write with QLowEnergyService::writeCharacteristic and read with QLowEnergyService::readCharacteristic. It all works well unless I have to read or write anything longer than 20 bytes in which case the data is cut at the end. Is there any way to either increase the write buffer size or specify the offset for writing?