Thomann TR 500 L Bb-Trumpet

I bought the trumpet as a beginner and had no teacher for almost a year, except a few online lessons to get me going. Nobody could assess the instrument, and as a beginner it is difficult to know if it is you or the instrument. Now I have weekly lessons since 6 month and my teacher concluded that the trumpet has quite some resistance, which does not make it bad per se, for the price, but also is probably too flat, when the tuning slide is completely inserted, making me struggle more to play in tune. I also had this doubt since the beginning, but at my level I could not be sure. If the mouthpiece could just be inserted up to 5mm deeper it would probably be good. We could measure a significant gap between the end of the mouthpiece and the leadpipe. I tried several mouthpieces, as to exclude that it was this one in particular. If Thomann agrees to take back the instrument and fix it at their costs, I will change my rating.