Adding an external using from meeting room

Hi Guys & Gals,

There is a number of posts on here about inviting external users to a Meeting Room invite and this is not my question.

My question is specific in the sense I have a Yealink Conference System, I create a meeting invite the attendees shows up and works no problem. 

However, from this unit I can add an internal user but not an external user as it is not showing. The account is a licences Teams Room standard and the organisation itself can already see outside (i.e. a user can add an external contact).

Hope that all makes sense, but looking to understand why it will not work from the unit.

Thanks in advance.