I have two of these routers. My first attempt to replace stock firmware had the same results as those of OP.
However, I managed to implement the instructions I found here on one of the two routers. It was a pain, and that was before I discovered LEDE, but I'm guessing the same procedure would work with the LEDE firmware instead of the DD-WRT firmware. I've not tried this yet (though I may soon). Hope this helps.
Myself, I'm now debating whether I should install LEDE or DD-WRT on the second router which is still running factory firmware. The fact that I've already successfully implemented mrjcd's instructions on one router argues in favor of me installing DD-WRT on the second router too, but I'm beginning to suspect that LEDE may offer more features than DD-WRT so I'm not sure. Maybe I should use LEDE firmware on the second router but just use mrjcd's instructions with LEDE firmware instead of DD-WRT. If I use LEDE on router2, then I'll probably overwrite DD-WRT on router1 with LEDE too, but I understand that carries significant risk of bricking the router. I want to use these routers with each other in WDS mode, and although I'm pretty sure that I could do this with one router running DD-WRT and the other running LEDE, my initial preference would be to have the same firmware running on both routers.
I'd enjoy reading others' thoughts on this decision.