In the past, SwiftSole used to rely on playing audio indefinitely to remain open in the background. Since this mechanism was faulty, SwiftSole now relies on location tracking to stay open in the background indefinitely.
Navigate to the Settings page.
Click the Background Mode selector.
When iOS prompts for permissions, select "While in Use"
Exit out of the app(but do NOT force close it) a few times until iOS prompts you again with a dialog asking if you want to let SwiftSole track location while in the background. Select "Always". It may take a few minutes/attempts for this prompt to appear.