Ultra 2 vs iSmoothRun

I used my iPhone 15 with my old Apple Watch Series 3 without problems. The iSmoothRun app worked perfectly. When I bought my new Ultra 2 it connected easily with my iPhone 15. All the apps from my Apple Watch Series 3 were copied on the Ultra 2. However, the iSmoothRun app on the Ultra 2 doesn’t connect with the iPhone. It uses different settings on the Ultra 2(e.g. miles instead of km), doesn’t show my equipment when I start a session (hence no option to select between my several running shoes), and doesn’t transfer the run data to the iPhone when the run is over (all of which worked with my Apple Watch 3). Although iSmoothRun says it can work with two different Apple Watches connected to the same iPhone (but not simultaneously) I’ve disconnected my Apple Watch 3 from my iPhone (now only the Watch 2 is connected with my iPhone). I also removed the ISmoothRun icon from my Ultra 2 and re-installed it. Nothing changed, however. Still the ISmoothRun app works on the Ultra 2 but does not connect to the iPhone. Any idea how I can fix it?