Interface freeze seems to occur randomly, but may depend on the traffic and number of connections handled by the device. The LAN/WAN phy links are still establised, but no traffic is forwarded on the copper ports. The device is still accessible from wifi, but the logs don't show anything except failed pppoe connection requests after that. The network functionality can be restored after issuing "/etc/init.d/network restart".
The factory default TP-Link firmware has the same issue! (posted to TP-link forum, waiting for publication)
Several people observed the bug in OpenWRT, and its forks: LEDE, and GLUON