To pair your contactless and chip reader wirelessly, you don't need to enter the number on the back of your card reader. Those numbers (the last four digits of your reader's serial number) just show up during the Select a Reader step for reference, so if there are multiple readers around, you can ensure you're connecting the correct one.
This support article has more info (and a video!) about the pairing process. And please write back if we can help you with connecting your reader!
