If you're not seeing your card reader show up as discoverable when attempting to connect it to your iPhone or iPad, here are a few things you can try:
Make sure your card reader is charged
If the card reader has a low battery it sometimes isn't discoverable, even if you're able to power it on.
Restart your iPhone / iPad
Oddly, this sometimes resolves discoverability issues.
Disconnect it from the iPhone / iPad it was previously connected to
If your card reader was most recently paired to a different iPad or iPhone, the reader may not show up as discoverable in the new device you're attempting to connect it to.
You may need to grab the other iPhone / iPad and disconnect the card reader from it so it can be discoverable. You can do this either from the POS app or from your iPhone / iPad's settings > bluetooth
Forget the Card Reader from the iPhone / iPad it was previously connected to
After disconnecting the device in settings > bluetooth you can also forget it, which may help the card reader be discoverable
Reset the card reader with a pin or paperclip
The card reader has a small hole in the back of the device that you could insert a pin or paperclip into. Insert and hold it for a few seconds until the card reader beeps.
Delete the Offset POS app and re-download it
Sometimes this resolves the issue, and may help you get onto the most recent version of the app if you're not current.