I'm unable to find an OpenWRT image for TD-W9970 v1 (no git commit for it, no wiki page for it, nothing in download section) so I will kinda assume it's not supported by OpenWRT.
Someone else asked something like this some time ago and got the same answer:
Trying to flash a firmware from another SoC that is from another manufacturer will result in a bricked router. That's why I said that 9970 is Broadcom and 8970 is Lantiq + Atheros .