Hmm , that device page is pretty patchy..
in any case , the window on most devices where the tftp connection can be made is very small - few seconds a most. On this point have you confirmed the tftp IP address is correct (its usually different to when the router normally boots) and your client machine has an ip in same subnet to match? Similarly, have you hard-set the mac address arp entry of the tftp (also usually different) on your client so it transmit the firmware image straight away when the router boots?
Theres a chance you have hit this issue too:
Update 2018-11-08: There has been a bug filed with regards to this behavior stating that in case of bad erase blocks prior to the factory partition OpenWrt uses incorrect/shifted offset addresses to access the factory partition. If this bug is confirmed this may the reason for the behavior reported by several users. Please follow the bug status over here: https://bugs.openwrt.org/index.php?do=details&task_id=1926
If so , youre likely gonna need serial to recover it
https://openwrt.org/docs/techref/hardware/port.serial