Need help please!

So I’ve been diagnosed with ST (Sinus tachycardia) but I’m unable to take meds for it. So I recently got a treadclimber tc5000 and have used it twice now but both times even going slow my heart rate gets up really high. I am not running, I’m going pretty slow, the highest speed on mine is 4 and at a 1.7 my heart is over 170. I’m used to hard work and walking a lot as I had a very strenuous summer job where I walked 4 or more miles a day. Does anyone have any suggestions for me? I could really use some help cause I really like my treadclimber and I want to get a little more active then I have been lately. I have to stop because I literally can’t catch my breath because of my heart. I was told by my cardiologist that even if I was running as fast as I could my heart rate wouldn’t get over 170. I wore a heart monitor for two week during the summer when I started experiencing chest pain caused by my rapid heart rate. Then my highest heart rate was 175, I’ll be seeing my cardiologist again in March and will be mentioning this but I’m not sure he can do anything for it. I can’t take meds for it because my heart rate drops to around 48bpm when I sleep and I have a resting heart rate of 64bpm. These pictures are what my Fitbit recorded while I was walking on the treadclimber.