Your phone becomes the controller, and the menu. Both devices must be on the same Wi-Fi. The game goes straight from your phone to this laptop; only the one-time introduction passes through a small relay, and it is deleted as soon as it is used.
This screen looks like a TV, and a TV has no camera, so the other way — your phone showing a code back to this screen — cannot work here. What usually fixes it: put the phone on the same Wi-Fi (not a guest network, which often stops devices reaching each other), then press START OVER for a fresh code.
Click a key, then press the one you want. If that key is already taken the two swap places, so nothing is ever left unbound.
PLAYED ON A COMPUTER
Arcfront runs in the web browser on a laptop or desktop — not on a phone or tablet.
arcfront.realweirdworld.com
Open it on your computer, then press PHONE PAD there and scan the code with this phone's camera. This phone becomes the controller.