These are the instructions I successfully followed on my C7 V2: https://wiki.dd-wrt.com/wiki/index.php/TP_Link_Archer_C7#Ins...

Here is the exact `factory-to-ddwrt` image I used (this will depend on which version you have): ftp://ftp.dd-wrt.com/betas/2019/10-15-2019-r41328/tplink_archer-c7-v2/