I have always been a “conventional operation mth man” . Until the Lionel Bluetooth came about. I like it a lot! It’s super simple, tons of fun, gives me all the functions I desire. Awesome throttle control at slow speed, smoke, forward and reverse without the engine cutting off then on, couplers for my switching. Really fun stuff! Then this happens as I fired her up. No sound control? And worse no couplers? Bummer. Any ideas? 
I think I had a similar problem when trying to use the app and the LC Universal Remote at the same time - at one point the loco was paired to the remote, but I wanted to control it with the app. App seemed to get confused and I believe I had a similar issue as you did. I also remember I wasn't able to change what locos the app was linked to.
I would power down everything on the layout, close the app, then try again. If that doesn't work, try restarting your phone. I haven't messed with the app in a while (much prefer the universal remote for LC and the Cab2 for Legacy), so I'm not sure if there are other options. If the engines are LC+ or LC+2.0 you may try running conventionally to make sure at least the sounds still function and isolate the problem to the app.
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