Although the TL-WR703N has a built in UART, as I have shown in my previous article, it’s rather difficult to access, and is useful as a serial console in OpenWrt. For the support of your own apps, it is far more convenient to simply plug in a cheap USB to serial adapter — this can be done without even opening the case. Since the TL-WR703N only has one USB port, if you are using EXTROOT, both the serial adapter and the USB flash drive can be attached to a USB hub: 
The TL-WR703N can easily supply the required current, so an unpowered hub may be used. To add support for USB to serial hardware, use opkg to install the appropriate modules. Basic USB to serial support:
opkg install kmod-usb-serial
FTDI (FT232) support:
opkg install kmod-usb-serial-ftdi
Silicon Laboratories CP210x (CP2102) support:
opkg install kmod-usb-serial-cp210x
Nanjin QinHeng Electronics CH341 (CH340G):
opkg install kmod-usb-serial-ch341
Prolific PL2303 support:
opkg install kmod-usb-serial-pl2303
Additionally, OpenWrt has packages for a slew of other USB serial adapters that I’m not familiar with. If you are using Attitude Adjustment (12.09) and its official repository, here are the other supported adapters:
kmod-usb-serial-ark3116 kmod-usb-serial-belkin -- Belkin
kmod-usb-serial-cypress-m8
kmod-usb-serial-ipw
kmod-usb-serial-keyspan
kmod-usb-serial-mct -- Magic Control Technology
kmod-usb-serial-mos7720
kmod-usb-serial-motorola-phone
kmod-usb-serial-oti6858
kmod-usb-serial-qualcomm
kmod-usb-serial-sierrawireless
kmod-usb-serial-ti-usb
kmod-usb-serial-visor -- Handspring Visor/Palm m50x/Sony Clie
After you install the appropriate kernel modules, your USB to serial converter will show up as /dev/ttyUSBx
[ 489.990000] usb 1-1.3: new full-speed USB device number 5 using ehci-platform
[ 490.100000] ch341 1-1.3:1.0: ch341-uart converter detected
[ 490.120000] usb 1-1.3: ch341-uart converter now attached to ttyUSB0