After spending quite some time trying to find a suitable new OpenWrt router I ended up buying the FB 4040 as I could get for around $50. Couldn't find anything with similar specs (2x usb/ac/250mb ram) for the money so I went for it.
Flashing this thing proved to be something that tried my patience to a point I didn't experience in quite some time 
After the umpteenth time trying to flash I managed to get into the boot loader ftp server but of course the macOS ftp client didn't work properly so I went ahead and compiled netkit-ftp from source and tried with that but after dozens of tries didn't managed to get back into the boot loader.
Long story short; I started to search the web for something that would make flashing easier and I found al little script that does everything. I have flashed the 4040 a few times with it and it went without a hitch even single time. It's called fritz-tools to be used for Gluon but it also works fine with image builder created images. It automatically found the .bin file that I put in the same folder and flashed it.
There are versions for Windows, macOS and Linux. Documentation and downloads here.
Hope this helps other flashing the 4040 without getting the urge to throw the router across the room 
(It might be nice to put this information in the wiki and/or fork this tool or something.)
