All the Same issues here. DLC was working but not CDLC. Ancient news feed messages from 2019 as well and told me my game was not legit, it is.
So what did work.... I turned off PC WiFi, started game, no issues with offline mode !
This was through the programs list for RS14, not Steam. I don't know if it matters, haven't tried Steam route offline. Or whether the manner of the originally installed game with hard disc or online through steam but mine was a hard copy. Hope that helps.