LoRaWAN is a popular low power wide area network technology widely deployed used in many scenarios, such as environmental monitoring and smart cities. Different applications demand various quality of service (QoS), and their service within a single network requires special solutions for QoS provision. We consider the problem of QoS provision in heterogeneous LoRaWAN networks that consist of several groups of devices that require different packet loss rate (PLR). To solve this problem, we develop a mathematical model that can find the PLR distribution in a LoRaWAN network. With the model, we show that the PLR can vary significantly, and it is wrong to consider only the average PLR for the QoS provision. Finally, we develop an algorithm for assigning modulation and coding schemes to end-devices that provides PLRs below the required thresholds.