A wrt54gl is to slow to run latest OPenWrt. You can try and flash to Chaos Calmer 15.05.1. Even 15.05.1 has bugs and should not be used on the internet. If using windows install winSCP. Upload the .bin file to /tmp Then use putty and do sysupgrade /tmp/file.bin If that will not work google about MTD. mtd write /tmp/file.bin linux
Or from the wiki.
cd /tmp/
wget http://downloads.openwrt.org/backfire/10.03.1/brcm47xx/openwrt-brcm47xx-squashfs.trx # The file must be the trx file.
mtd write /tmp/openwrt-brcm47xx-squashfs.trx linux && reboot