That's good to hear. I have whipped up a 15.05.1 image which you can find here. If my domain does not resolve, try with e.g. Google DNS or OpenDNS servers.
If my assumptions are right this image should boot on your device. If the WAN LED you're talking about does not function, however, you need to 'poke at the GPIOs' like I was told in #openwrt to find out what GPIO handles the new LED, which we can then add to the OpenWrt code. I haven't done that myself yet (I just tested a bit though) - you can find info on hat here.
You first need to export the GPIOs, this e.g. will try to do this for GPIOs 0 through 30. Judging from the code for the TL-WR740N, that should be enough to start with:
for I in 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30; do echo $I ; echo $I > /sys/class/gpio/export ; done
This should give you new files in /sys/class/gpio, like this:
root@OpenWrt:~# ls /sys/class/gpio/
export gpio10 gpio12 gpio13 gpio14 gpio15 gpio9 gpiochip0 unexport
Maybe someone else can help out on how to proceed further, to probe those. I guess you'd need to try the other commands from the Elinux page and watch how the LEDs (or the device) reacts to it.
LEDE 17.01 RC2+ on D-Link DIR-860L B1 :: Netgear WNDR3700 v1 (2x) & v2 (1x) :: PC Engines APU2 :: Ubiquiti Unifi AC Pro (2x) :: TP-Link TL-WR1043ND v1 (1x) & v2 (4x) :: TL-WR841N (2x) :: LEDE trunk on Asus WL-500W (wl) :: OpenWrt Attitude Adjustment 12.09.1 on Asus WL-500G Deluxe