I have same problem with my router tl mr3420 v1.3
I flash my router with openwrt-ar71xx-tl-mr3420-v1-squashfs-factory.bin and then i brick the router
Someone write step by step comands in telnet how i can back original firmware
I read in internet every router have different comand to erase copy and boot the original firmware
My question is what comand is write and then i see the numbers to erase copy and boot
My example to comands on other router
ar7100>
ar7100> erase 0x9f020000 +0x3c0000
ar7100> tftpboot 0x81000000 code.bin
ar7100> cp.b 0x81000000 0x9f020000 0x3c0000
ar7100> bootm 0x9f020000
What comand i write on my router
Someone who has done original firmware on tplink mr3420 ver 1.3
write comands here step by step , because I do not know much
and someone upload the pic on the serial cable because i understand 1 or 2 resistor 10 kOm i put on my router
and rezistor between vcc - Tx or vcc -Rx or two