Low image quality
This is a quality issue with your endoscopic camera. The app itself allows you to work with up to Full HD resolution (1920x1080 pixels). The solution is to replace the endoscopic camera.
 
There is no image at all
In 99% of cases, this is a problem with your OTG USB connector. The solution is to replace your OTG USB connector with another one or clean it well (remove a dirt).
 
I can't find the recorded video files in my smartphone
You need to select a public folder to save your files. To do this, select in the app: Settings - File folder setup - Public storage. Then you need to choose a public folder in your smartphone like "Movies" or "DCIM".