Hi,
When considering on whether RDS CAL or External connector, you shall check the user numbers and confirm the payment with your sales.
External Connector is paid by server while per user/device CAL is paid by exact number you needed.
https://download.microsoft.com/download/3/D/4/3D42BDC2-6725-4B29-B75A-A5B04179958B/Licensing-Windows-Server-2012-R2-RDS-and-Desktop-Apps-for-RDS.pdf
Reference link of similar thread.
https://social.technet.microsoft.com/Forums/en-US/35e32537-708e-4839-8e84-1b796f7dcbcf/rds-licencing-ec-vs-per-user?forum=winserverTS
Please also check with VLSC on the detailed price and explain your requirement to get a most cost-effective plan.
VLSC contact:
https://www.microsoft.com/Licensing/servicecenter/Help/Contact.aspx
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Hope this helps and please help to accept as Answer if the response is useful.
Thanks,
Jenny