Download one of the TL WR 841 V12 versions from here according to your router, EU or US: https://downloads.lede-project.org/releases/17.01.4/targets/ar71xx/generic/
Log in to the router GUI, default username / password is admin / admin .
You can flash the firmware from the router GUI > System Tools > Firmware upgrade. You probably rename the filename to a much shorter one if you get a message to choose the file (again). Make sure the file extension remains the same.
Once it's uploaded and the router rebooted itself, you can access it on 192.168.1.1 from the browser. If your cable modem/router is on the same IP address (you'll realize this if you'll have connectivity issues), then you have to go Network > Interfaces > LAN > Edit and set a new IP address of 192.168.20.1. (192.168.1.x addresses would not resolve the problem) .
Download putty.exe (https://the.earth.li/~sgtatham/putty/latest/w32/putty.exe) and you can open the router (while it's connected to the internet) on its IP address. Enter root for the username and you'll gain access.
type the following:
wget http://hotspotsystem.com/firmware/openwrt/TP-LINK/tplink-tlwr841/15.05.1-tl-wr841n-v10-sysupgrade.bin (YOU CAN COPY THE LINK from here: http://hotspotsystem.com/firmware/openwrt/TP-LINK/tplink-tlwr841/ )
sysupgrade -F -n 15.05.1-tl-wr841n-v10-sysupgrade.bin
Then the router will come up on 192.168.20.1 and you can follow with running the setup script (from SSH the same way as you upgraded the router) from the Control Center right away.