You can easily get temperature readings from external Dallas DS18S20 sensors (1-wire protocol) on your TP-Link WR703N.
After you got hardware part sorted now you need to setup software part. Main tool for reading temperature from 1-wire sensors is digitemp. Digitemp uses bit-banging to emulate 1-wire protocol over serial port.
opkg install digitemp
You also need usb to serial kernel module (driver) so install that also:
opkg install kmod-usb-serial-cp210x
After digitemp is installed you need to search 1-wire bus for all connected sensors, each sensor has it's unique address so is has to be discovered and added to config file with this command:
digitemp_DS9097 -s /dev/ttyUSB0 -i
Now you can read temperature from your sensor:
digitemp_DS9097 -a -q