Chaos Calmer is past its EOL date and is both not supported and known insecure.
Instructions for current releases are often not supported by older releases, especially that far in the past.
Further, the TG799vac does not seem to be a supported device, but running the OEM's custom firmware. Even though it may say "OpenWrt" on device, as OpenWrt is relatively freely licensed, OEM often take it as a base for their firmware and modify it as they see fit. The end result is often incompatible with the upstream OpenWrt source.
There are several posts that I recall about Technicolor's unwillingness to supply GPL sources.