Even though it is marketed as a “3G travel router”, the TL-WR720N does not include a 3G modem. It simply means that the OEM firmware and GUI support a certain range of USB 3G modems. OpenWrt supports USB 3G modems, too. The router is powered through either a AC source (100-240V, Chinese-style foldable plug, i.e. two prongs without hole), or a micro-USB socket stub (5V).
The domestic Chinese/Asian versions and those for the European and American markets differ significantly. Only the rectangular version available from China has been tested, so far.
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.).