Did you get a resolution to your charging issue? I have a Model 3 and it works perfectly. I just leave it set to be fully charged by 6am. It usually starts charging for a minute and then decides when to charge. I have seen it stop and start which I assume is based on low/high grid availability, but it has always been charged by the end time. I leave   the charge limit set to 100% in the OVO app as the charge limit you set in the   Tesla app will stop the charge so it’s then the same process charging at home or at a charger away from home. It’s also then straightforward for my partner to charge without knowing anything about the Charge anytime app. I assume the Charge Anytime app checks the Tesla charge limit when estimating how long you need to charge the car. I do like that you aren’t restricted on times you get low cost charging as you could set the car to be ready at anytime day of night so it’s very flexible. 
 On security, the app does know whether you are charging or not and where you are as it will show whether you are charging when  away from home, in which case you obviously get no credits. 
 If I had a compatible charger (mine is a Hypervolt) then I would use this but I’m pretty happy with the process as it is.