Thank you so much for building this!
I was lucky enough to you have factory firmware 1.1.5 and was able to flash from factory GUI!!! Not sure if you can downgrade if you are on 1.1.7 or 1.1.9. 1.1.5 no longer on Linksys site. Found this:
Older firmware also might be in your alternate boot location which I recall having read loads after 3 failed attempts at normal boot. Power off 10 seconds into boot maybe???
Otherwise, you probably have to go serial cable...
Another reference thread:
https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=280327&postdays=0&postorder=asc&start=15
Hope that helps.