Q. External users are attending a Teams meeting but cannot participate in the chat. How can external users be allowed to join the chat?
A. Please create the meeting without specifying a channel.
If you specify a channel when creating the meeting, only members of that channel can use the chat, and external users cannot participate in the chat.
For meetings that include external users, create the meeting without specifying a channel.
Reference: Is it possible to add accounts from external organizations to chats or teams?