I bought a TP-Link with a broken Wan Port, I updated the Firmware to Barrier Breaker 14.07/Luci Trunk in order to attempt to convert a Lan port into a Wan port. I am completely new to Openwrt, so I apologize for my ignorance. I have searched the forums and found a few solutions, but each one tends to make my Router non-responsive and I have to reset it. I would appreciate help if anyone is willing to lend a hand.

(Below are the directions that led my router to become non-responsive)

Thank you tomaxsas, confirmed success to substitute wan port to one of the LANs ports.
on TL-wrt740n v4.24 , it goes like this:
install openwrt , link http://wiki.openwrt.org/toh/tp-link/tl-wr740n ,  find yours.
log to 192.168.1.1 , user root , password is empty. Dont set root password , if you wont to reset router.
go to "network" tab, under that "swich". Check, if not,  "enable Vlan". 
You will see what port is connected to computer "100baseT", other should be empty. Insert internet cable in LAN port, and one icon will change to "100baseT" or something. That port set to off in first vlan. set cpu to taged. rest remain untaged.
Add second VLAN. set cpu on the second VLAN to taged, set same internet port (off in first vlan) to untaged, rest are off.
Save and Apply.
Go to "Interface" tab. On WAN network go to edit , find "Physical Settings" tab, change from ethernet(WAN) to VLAN interface (eth0.2)  (decheck Ethernet, check Vlan...). Leave other settings as is. Save and apply.
Go to "Interface" tab. On LAN network go to edit , find "Physical Settings" tab, change from ethernet(LAN) to VLAN interface (eth0.1)  (decheck Ethernet, check Vlan...). Leave other settings as is. Save and apply.

READ this, what to do -if you do something wrong- to reset router :  http://klseet.com/index.php/tl-wr1043nd-ver18/reset-failsafe-revert-to-factory , I do it many times wink
Go to system, then Reboot.   
ps. in section system - led configuration , first led is "WAN", change if you want, device to "eth0.2" to have visual on internet. (if you later forget what port you set to be WAN)
Thanx tomaxsas again, and be careful. -Prviuredu

Port 5 Is the Existing Wan Port

(Last edited by PhilDozer on 14 May 2015, 11:13)

In your second picture (switch configuration), for example to make port 4 the new WAN port, underneath port 4 change the box on the first line (VLAN 1) from "untagged" to "off".  This makes the port no longer part of the LAN.  Then change the second line from "off" to "untagged".  This makes it part of the wan.  You can turn the bad port (5) off entirely though this should not be necessary.

This is the ONLY change you need to make, other than plugging your ISP into port 4 instead of "WAN".  The networks are already set up.

Note that switch port 6 is physically connected to CPU port eth0 though it is not labeled that way on the page.  See the WR1043 wiki about the V2 switch arrangement.  A V2 like you have is quite a bit different from the V1.
http://wiki.openwrt.org/toh/tp-link/tl-wr1043nd

(Last edited by mk24 on 12 May 2015, 02:55)

Thank you very much! I kept changing the CPU Port to "tagged" per the instructions and that what was causing me to disconnect from the router.
I'll add "Solved" to the title of the thread. Thanks again!

(Last edited by PhilDozer on 14 May 2015, 11:12)

I tried the method posted by tomaxsas but couldnt get it to work.After changing the physical settings of wan to vlan eth0.2 I couldnt log into my router anymore.can You please guide me.I am new to this and getting all confused.how many interfaces should there be?i have three,wan,lan (bridged)and a disabled wan6.but when i tried to log into router using failsafe I couldnt because the physical address had changed.it wasn't 192.168.1.1.please help?

hello guys i faced the same problem with my TP- Link 740N, i couldn't know what i supposed to do to change my LAN port into WAN and specify WAN configuration so could anyone help me with please,
i am using Barrier Breaker V14.0.7
thanks in advance

Complete guide how to use LAN port as WAN (compiled from all of the above).
-This guide assumes that you already have OpenWrt installed
-I've done this on WR740N
IMPORTANT NOTE: LUCI Web Interface has bug with port ID in switch section (Checked on v4.20), LAN Port number on router=Port number in LUCI switch 1=2, 2=1, 3=2, 4=1

To set LAN4 port as WAN

Network>Switch
VLAN          CPU         Port1          Port2          Port3          Port4
  1               tagged         off           untagged   untagged    untagged
  2               tagged       untagged      off              off               off

Click Save (not Save & Apply)

In Network>Interfaces>LAN>Physical Settings
to make vlan1 LAN port uncheck eth0 and check eth0.1 and keep wireless network checked also
click Save (not Save & Apply)

In Network>Interfaces>WAN>Physical Settings
select eth0.1
Click Save & Apply

Probably eth0.2 should be used for WAN. Thanks anyway for guide. Very usefull.

(Last edited by d1188636 on 20 Mar 2018, 15:11)