If you're having issues connecting your Square reader, you can go straight to the source and read what Square recommends here: https://squareup.com/help/us/en/article/5673-square-contactless-and-chip-card-reader-troubleshooting.
If you continue to have trouble with your reader after trying the above steps, you can reset your reader:
Charge your reader for at least 20 minutes.
While the reader is charging, use a small object like a bobby pin, pen cap, or paper clip to firmly press and hold the button on the reader for 20 seconds.
After 20 seconds, release the button and wait 10 seconds. Do not let go of the power button until after the red lights have stopped flashing.
Press the button again to turn the reader on and observe any lights on the reader.
If the reader lights turn on, you’re all set. If the lights don’t turn on, please try again. Make sure to firmly press and hold the reader’s button for a full 20 seconds. Do not let go of the power button until after the red lights have stopped flashing.