We have a MR33 AP delegating IPs from a DHCP server. The laptop I'm using can ping the entire network, however nothing on the network seems to be able to ping back. We can reach the AP just not anything that is sitting behind it.
Try disabling your Windows firewall and see if that works. Also, check if you have any layer 3 firewall rules configured on the Wireless > Firewall and traffic shaping page.
This is very good advice. Windows firewall likes to block external pings.
You can test for Windows FW issues by confirming the AP is pingable by another device on the same SSID. Then try to ping the AP from the problem child device. If problem child can ping AP, but AP can't ping problem child, it's usually software firewall.