Note: Many of these routers are marketed as a “3G travel router” but none actually include a 3G modem - the marketing term rather means that the OEM firmware supports USB 3G modems because it contains drivers for USB modems! Ignore that, because with OpenWrt ANY router with USB supports 3G hardware…
As with most TP-Link Routers this one can be flashed using the TP-Link Webinterface. The webinterface is chinese, but http://192.168.1.1/userRpm/SoftwareUpgradeRpm.htm is the firmware upgrade page, where you can upload the firmware file. Note that you cannot navigate to this page directly, and need to come from the page http://192.168.1.1/userRpm/DateTimeCfgRpm.htm. An easy way to navigate the pages is by hovering over the menu links and viewing the URL in the status bar of your browser.
A custom image with ipv6 support: radvd, wide-dhcpv6, 3g stick support, made for RCS-RDS Fiberlink dual stack PPPoE service,
but should be okay for static wan settings on other ISPs:
Forum member pepe2k made a modification of U-Boot 1.1.4 for Qualcomm Atheros SoCs based devices (the project is still being developed, so new devices and SoCs will be supported in the future). Up to date information, binary images and sources can be found on official GitHub repository.
This modification started from wr703n-uboot-with-web-failsafe project, but supports more devices, all modern web browsers, has a lot of improvements and other modifications (like U-Boot NetConsole, custom commands, overclocking possibilities etc.).