Hi Jaguar76 - welcome to the forum!
First things first: the ARC Pro powerbase is designed work fine in both analogue and digital mode without the app being connected. The small switch on the side selects analogue on the left and digital to the right. I'd recommended labelling the two positions (I always forget...). The way you tell digital mode is that a non-chipped car will hum loudly when placed on the track and won't move. In analogue mode, the same non-chipped car will be silent when stationary and move only when you press the throttle.
If you car keeps stopping, I would make of a note where it is stopping. If it is the same place(s) each time, then check the track condition and connections. There are some very good maintenance tips here: www.scalextric.com/uk-en/support/track-maintenance
You say that the main light on the powerbase is flashing orange - this is what it should do when the powerbase is on but there's no bluetooth connection (see page 5 of the Quick Start Guide). If it then goes solid green, it suggests there is a device connected by bluetooth to the powerbase, which is very odd if you do not have the app open on a device nearby. If it has gone solid red, it would indicate a short circuit. I wonder if you are getting green instead of red and you have a short - this would cause the cars to stop and the LED to change at the same time... Again check the track and the braids on the cars to see if any metal is bridging the two rails on the lane you are using. Does it happen on both lanes or just one? Does it happen with all the cars you have, or just one?
Another question: what is the model number and the rating (volts and amps) of your power supplies? ARC Pro requires two Hornby P9300 AC adapters providing 15 volts and 4 amps each. To run analogue, only one power supply is required, but it must have the 4 amp output. A less powerful power supply (or pair of supplies) could cause these problems.
The bluetooth connectivity almost certainly isn't the issue, in my opinion. However, if you want to check this, I'd recommend trying an iPhone or iPad to check the connection - the Apple devices are pretty stable with the ARC app. There is a whole thread about device compatibility here: www.scalextric.com/uk-en/forum/arc-app-device-compatibility/
To save you trawling through twelve pages of posts, I'll summarise to say that Android device compatibility can be patchy. There are reports that some (older) Samsung devices work fine, but other newer Samsung models do not. A regular ARC user that I know reports on that thread that he has successfully run the lastest ARC app with Samsung S7, S8 and S9. So that's hopefully good news.
How the app should connect is that you first power up the powerbase, then launch the app. The blinking orange light on the powerbase will eventually turn a solid green when connected - this can take 5-10 seconds. A manual search and connection is not recommended. If you are switching between digital and analogue, I strongly recommend you do this with the app shut down and the light on the powerbase flashing orange.
Finally, as the ARC Pro powerbase was bought 'used', I would suggest asking the vendor if they had any problems with it. Personally, I would be nervous buying a complex powerbase like ARC Pro second-hand.
I hope that helps.