I think since your server is hosting a non-Microsoft program not utilizing the underlying operating system from the client side that as long as you have the default cal (1 or 5, etc) you should be fine. If you want to know the truth, I would recommend you reach out to a microsoft license reseller (multiple recommended) to get their input.
Technically you need to purchase the Windows External Connector license. This license covers all remote uses connecting to services on the server. This is the same license that you buy so that you don’t have to have a CAL for every user connecting to IIS.