I don't have a TP-Link router but a quick google gives this
page. In this case you want to allow pings from the WAN port but it probably defaults to blocking them. If that option is not available on your router there may be something similar that talks about WAN ICMP echos or WAN Pings.
I have also seen a couple of posts that suggest TP-Link might have removed the option, if that is the case then you may not be able to easily set this up (for example BT on their latest hub do not give any ability to respond to pings and so it cannot be used with BQM).