Hi,
You don't need serial flash, editing the firmware file cautiously with a hex editor will get it up again. I've found a post somewhere herein ago and followed to enter the "wrong" product id as hex code into the flash image and then I could flash it with tftpd32 . After this was successful, openwrt booted again and I could do a sysupgrade -f (force) flash to get the original fw image in, containing the right product id. After the two flashes I no more had problems with brick and I can normally upgrade openwrt through the web ui.
I think I found it here again : Can't get my archer c7 unbricked.. been trying all weekend
The serial console could only help in getting the product id from the uboot output that you know if ot expects a to-flash file to have 0x00000000 or 0xffffffff
The original post has the content to be put in by hexedit, Can't get my archer c7 unbricked.. been trying all weekend
But it's about restoring a more severe damaged router. If you just have a brick after flashing which occured to me to multiple devices when upgrading from 18.x to 19.x , then hex editing and uploading this via tftp works.