Using bluetooth on an old Watch App

I have an old watch app called Zepp Tennis. Part of its feature is allowing access to a sensor on the end of a tennis racket which then displays the tennis racket’s speed right on the watch. It requires accessing the watch’s bluetooth to do this. Due to a lawsuit this company is now out of business and it seems unlikely that they’ll change this app at all. I have a problem where it seems as if this feature is not possible on WatchOS6, because it doesn’t ask for Bluetooth permission correctly. But it _is_ possible on WatchOS5. I was able to get it to work on an old watch right before I tried to replace the battery and it bricked. If anyone else can test if this works on WatchOS6 please let me know. How can get I simply give this outdated app the necessary permission to use Bluetooth so that it works on WatchOS6? To be clear, the watch needs to connect to the sensor directly - not through the phone. It is impossible to add a new watch to an apple account that uses WatchOS5. Apple will always auto update it to WatchOS6 before adding the watch, annoyingly.