Here is a short troubleshooting process to attempt a new connection:
Make sure that Perifit App has Bluetooth authorization on your device. Go to your phone or tablet settings > Security and Privacy > Bluetooth > Check that Perifit app is enabled to use Bluetooth.
Disconnect other Bluetooth devices currently connected from your phone's settings.
Close the app completely (iOS: tap and maintain the app page on your screen + swipe up to remove it. Android: (double) tap the central button and tap the closing arrow).
Turn on Perifit (keep it in your hands). The LED should flash. If not, let me know.
Re-open Perifit app
From the app tab "Today", tap "Start". The app will attempt to connect to your Perifit.
Is the connection successful?
If not, try restarting your phone and try the above process again. This will help reset the Bluetooth parameters on your phone.