Thank you for your help. I have tried several methods now, but I admit I have given up right now.
I have tried to flash several images including DD-WRT via TFTP, but all failed. I decided to take the risk and try a second router (I had ordered 5 for my project) and immediately flash LEDE via TFTP, instead of the web interface. It turned out that this router is probably not accepting images at all via TFTP. At least I transfered the LEDE image via TFTP, then waited 15 minutes, then power-cycled the router - it still had the stock firmware. Tried another 2 times with the same result. Probably there is some magic required, but I don't know it 
So I gave up here and decided to try TP-Link Archer C20 instead. Hope these work better.
I will try serial recovery when I have time. Unfortunately, this revision doesn't have any holes, just soldering pads. I'd have to drill some really thin holes myself. And it has two connectors looking like the one in @JTRealms picture, one labeled "DJ2" and the other one "DJ6".