My guess is that you installed a newer version of the stock firmware with an open source firmware unfriendly signature check. dd-wrt gets around that better than OpenWrt in my experience. I know this sounds crazy, but try installing dd-wrt again, then revert from dd-wrt back to an older version of the stock OEM firmware. Look for a link on the "revert to stock" sticky thread on the dd-wrt web-site for one that is old enough.
Another possibility is to check that you are using firmware specific to the region of the TP-Link hardware/firmware, e.g. "C7 V2 (US)" and "C7 V2" may be the same hardware, but the stock firmware is slightly different and requires slightly different open source factory replacement firmware binaries.
These are the two things that tripped me up when I had Archer C7's.