Download firmware from https://advancedtomato.com/downloads/router/e2500v3
add connnector on board near blue WPS button and connect RX,TX and GND to TTL Serial usb
minicom connect to /dev/ttyUSB0, 115200,
Ctrl+C
Work upload by TFTP client, but need classificate what parttion you update
https://www.dd-wrt.com/phpBB2/viewtopic.php?p=895421#895421
flash -ctheader : flash1.trx
# after this command run tftp
# tftp -m binary -c put [firmware]
nvram erase reboot