According to the Microsoft documentation4, external access settings in the Microsoft Teams admin center are used to control how users in your organization can communicate with users from other domains that use Teams or Skype for Business. External access settings do not affect how guest users in your tenant can use cameras during Teams meetings.
To prevent guest users in your tenant from using cameras during Teams meetings, you need to create and assign a custom meeting policy that has the Allow IP video setting turned off. You can do this by using the Microsoft Teams admin center or PowerShell. For example, to create and assign a custom meeting policy by using PowerShell, you can use the New-CsTeamsMeetingPolicy and Grant-CsTeamsMeetingPolicy cmdlets.