Just a RoC key will grant both Classic RoC and Classic TFT in the Reforged client.
Do they show in your “Games & Subscriptions”>“Classic Games” like below?
If not, either you claimed it on a different Battlenet account sometime in the past, or the key was used by someone else.
Where did they say it’s your fault? It also hasn’t stopped working. Without Reforged, you need to prove to Battlenet that you own the original WC3 and that is done by linking a RoC key to your Bnet account.
You can check in your Classic Games section in your Account Management. (See the screenshot above.)
See here:
The same thing happened to me way back in the day when I tried to link mine. Customer Support was able to link them for through a support ticket (basically what Drakuloth said above me). I recommend either the Live Chat or Callback. It’s much faster.
You don’t need it for Reforged. The RoC key alone will unlock both RoC and TFT.