Hello dear community - sure this has been covered before but it was intensely difficult to find, and since I got help via email from Matthias Schiffer regarding this solution, I decided to post it publicly.

TP-Link, as BrainSlayer points out in post viewtopic.php?id=63123 in the forum, has a verification, of sorts, that results in the dreaded 18005 error when flashing from stock to openwrt or LEDE.

One fix was to add a USB with a script, and have it run via modification of the WIFI SSID (too complicated but actually rather brilliant),

Another fix, rather time consuming, was to flash dd-wrt (with the region fix incorporated), flash back to stock (pre-region-code-validation), and then flash OpenWRT.

And this new Fix, which is actually here >

git lede-project org/?p=lede/neoraider/staging.git;a=commitdiff;h=72fd20205fe26b434b6a292926ccc569c6b1ce0c

Which all it does is add this line:

IMAGE/factory.bin := append-rootfs | mktplinkfw factory -C US

right before "endef" inside target/linux/ar71xx/image/tp-link.mk

Example Before for TL-WR710N:

define Device/tl-wr710n-v1
  $(Device/tplink-8mlzma)
  DEVICE_TITLE := TP-LINK TL-WR710N v1
  DEVICE_PACKAGES := kmod-usb-core kmod-usb2
  BOARDNAME := TL-WR710N
  DEVICE_PROFILE := TLWR710
  TPLINK_HWID := 0x07100001
  CONSOLE := ttyATH0,115200
endef

Example After:

define Device/tl-wr710n-v1
  $(Device/tplink-8mlzma)
  DEVICE_TITLE := TP-LINK TL-WR710N v1
  DEVICE_PACKAGES := kmod-usb-core kmod-usb2
  BOARDNAME := TL-WR710N
  DEVICE_PROFILE := TLWR710
  TPLINK_HWID := 0x07100001
  CONSOLE := ttyATH0,115200
  IMAGE/factory.bin := append-rootfs | mktplinkfw factory -C US
endef

THAT'S IT!

The more in depth explanation from Matthias is:

"
Correct.

The default IMAGE/factory.bin definition used for TP-Link devices doesn't
set a region code (the default is defined in Device/tplink, which is
included by Device/tplink-nolzma, which is included by Device/tplink-8m,
included by Device/tl-wr710n-v1). The arguments '-C US' are passed to the
mktplinkfw tool (through the Build/mktplinkfw definition, but that's a bit
more complex...)

"

Time consuming? Not really, far easier and less chance to goof.
Besides, NO command lines (or similar) for the 75-90% that prefer not to go that route mostly because it's too complicated and/or confusing.

Use the factory-to-ddwrt-us bin to get rid of the 'locked' TP_Link F/W (from Oct or Nov 2015), then the wdr3600v1_webrevert (your model number) bin to revert back to the origibal TP-Link F/W and lastly, your choice of the replacement Openwrt or dd-wrt F/W.