So Kong from the DD-WRT forums used to provide builds for DD-WRT that I used for my MVEBU Linksys 1900acv2 router. He apparently doesn't anymore, which is fine, brainslayer builds are fine for me, but I wanted to keep the bins he had uploaded for reverting to stock.
I figured I'd post them here in case someone else is looking.
While you shouldn't need these for your 1900ac router if you followed the "FAQ" thread response I wrote, there may come a time you do need it. I tested it before on my acv2 from Kong but I actually got these from a different mirror (since kong deleted them) so they are not tested. I may test them someday. Until then, use at your own risk.
This is a single zip archive with all of the factory firmwares for 1200ac, 1900acv2 and 1900acs, which in theory the acs one should also work with the 1900acv2. Sorry, I don't have them for any other router, but you can check out this thread for details if yours isn't listed here which I can confirm works for the later 3200acm unlike testing these which I haven't done yet. Been busy working but perhaps someday I will. If you can comment below your results though that would be appreciated!
Of course.... if you completely brick your router, you can always go the route of a usb-to-ttl cable.
Anyhow, here is the download link.