You may use the arp-table, or DHCP-leases. Not a perfect solution, maybe it’s enough?

List arp-table

arp

List DHCP-leases

cat /tmp/dhcp.leases

… and combined

for ip in $(arp | grep -v IP | awk '{print $1}'); do 
    grep $ip /tmp/dhcp.leases; 
done

https://serverfault.com/questions/226046/how-to-get-a-list-of-the-connected-wifi-clients-in-openwrt-10-03