Realize that ARP is only going to show devices that are participating in IP, and have recently been communicating with/through the router.
I usually do a ping scan of the network and then check the ARP cache. If my own computer is in the same network to be tested, then just ping scan the LAN and check your own computer’s arp cache.
Even then, some device that’s not speaking IPv4 (say an IPv6-only device) would never show up in an ARP scan.