TFTP boot didn't work for me. It turned out that the product id doesn't match.
Had to connect to console.
1. The pinout is as follows. The warning on this page (https://wiki.openwrt.org/toh/tp-link/tl-wdr7500#tftp_recovery_de-bricking) itself is wrong.
2. I used the Openwrt Snapshot image. I tried the official image from TPLink website but that didn't work. I didn't try the "cut" process described in the above link.
3. The command is as follows
type "tpl" really fast at boot time to stop the autoboot
tftpboot 0x81000000 [name of your firmware file].bin erase 0x9f020000 +f80000 cp.b 0x81000000 0x9f020000 0xf80000 reset