(edited by Archangel.8243)
HoT is registered, but cant play
G2A is reseller, so you’re buying from random people, hence why they say you shouldn’t. The problem in doing that is that the original purchaser still has control over the key and can easily revoke it, causing your account to be banned.
You should immediately receive the level 80 boost and be able access HoT’s story after upgrading. Restart the client if you haven’t. You’ll need to continue talking to support if it’s still not available after a few hours.