Isso é copiado da página do OpenWRT referente ao seu roteador :
Wired stations cannot ping each other
This is a known bug: "switch eth0(Atheros AR8316) no traffic between ports possible" Based on the comments from the bug you can try these commands:
swconfig dev eth0 vlan 1 set ports "0 1 2 3 4 5" swconfig dev eth0 set apply
If these work you can add the following lines to your /etc/config/network:
config switch option name 'eth0' option reset '1' config switch_vlan option vlan '1' option device 'eth0' option ports '0 1 2 3 4 5'