Scp command
From DD-WRT Wiki
Occasionally the DD-WRT webif will decline to accept a valid firmware. SCP is a good alternate as long as you've enabled sshd on the 'Services' page.
scp -rp local_firmware_name.trx root@192.168.1.1:/tmp/remote_name.trx
Login with ssh and
mtd -r write /tmp/remote_name.trx linux