I'm Brazilian too.
So, there is no TFTP recovery mode on firmware build 180202 and 180529. Both of them are factory versions here in Brazil.
However, TP-LINK has released firmware build 190404 (Agile Config 2.0 compatible) and the uboot has been updated. Now, the TFTP recovery method (TFTP server @ 192.168.0.66, tp_recovery.bin, like others TP-LINK) works fine.
So, if you want to install OpenWRT you don't need to use the serial/TTL method, just upgrade it to build 190404 first and do the TFTP method.
Download the 190404 build here https://gofile.io/?c=HXdgSF
To the non-Brazilians: test this procedure too, but you've got to ask TP-LINK for the new firmware, or check the TP-LINK website.
edit: if you want to restore the original firmware, remove the first 512 bytes of the firmware, save it as tp_recovery.bin and do the TFTP recovery. You can use this method to cross-flash firmware from others regions. The bootloader is not overwritten by TFTP method, the only way to replace the bootloader without TTL cable or external programer is using original flash update by web ui.