[QUOTE1Gbps broadband connection and only getting about a quarter of the speed, does it make any sense? ][/QUOTE] That's normal without hardware NAT support. You have to think about another router...

Have you tried settings TP-link like AP, without WAN and DHCP Settings? http://www.tp-link.com/en/FAQ-417.html

Sorry I don't know any method in alternative firmware. Please direct your question here: https://forum.openwrt.org

The cable that connects modem and routers is original or not? What kind of WAN settings do you have (Dynamic, Static, PPPoE etc)? This update was properly executed? Do you have access to router...

No problem, greetings from Poland!

You have to send those informations here. May be it's some bug in firmware - http://ticket.tp-link.com/index.php?/Core/Default/Index - I can hear about it first time.