I know the question is about an old hardware, but we have a lot of these guys around and sometimes in a update, the APs may brick, but in general can be saved.
Unfortunately some of the access point don't come back with the normal installation, so I wanna know if there is a way to install the OpenWrt using the JTAG port using a USB Blaster.
Thanks for the attention.
PS: Sorry about the English, isn't my first language.
You might find some information at https://openwrt.org/toh/tp-link/tl-wr740n#debricking about how TFTP might work.