Due to increasing of economic and scientific activity in the high latitude regions interest to information tools has been raised. One of the basic ways of exchanging information is the propagation of high-frequency (HF) waves, which requires knowledge of the state of the ionosphere. One of the main ways to describe this state is to use models. In the present paper, the most widespread model IRI2016 is used, however it has not been adequately tested in the high latitude regions, especially from oblique sounding and during disturbances. Results of testing the state of the ionosphere and the characteristics of HF propagation are presented for the period of September 2017, the month with the strongest disturbance at the end of the year. Two methods: analytical and numerical modelling, based on ray tracing, were used to determine the maximum usable frequency MUF on the paths of Gorkovskaya-Lovozero and Cyprus-Lovozero. A special feature of the study is the use of vertical and oblique sounding data with a 15-minute resolution. It is shown, that during the period of September 6-8, the MUF experienced a positive disturbance. The model values corresponded to the experimental magnitudes rather well. The possibility of using the analytical method for obtaining the MUF is confirmed by ray tracing. It is shown, that for monitoring of ionospheric conditions along any path it is possible to use the total electron content measured by means of navigation satellites GPS, GLONASS.