#2 - July 7, 2018, 4:58 p.m.
You should be able to get the Trade channel back by resetting the chat to default.
Type /resetchat in the chat bar and press Enter, then click Accept.
You can also manually rejoin the Trade channel while in a major city, like Orgrimmar or Stormwind by typing /join Trade .
If you still have the same problem, exit World of Warcraft and follow these steps:
1. Go to your World of Warcraft installation folder
2. Double click on the WTF folder
3. Navigate to your World of Warcraft account folder, then realm, then the affected character
4. Delete the chat-cache file