Help me understand Microsoft licensing (... yes, I know, lol)

We all know that MS Licensing is confusing as hell, etc. I think it's funny too, but now I need to try to explain to some people how much a solution is going to cost because of its requirement to run on Windows and MS SQL. I work for a (non-profit) internet provider. Everything that the company is currently doing is FOSS-based and running on one flavor of Linux or another.

I can figure out the Windows Server OS licensing ([this site has been supremely helpful](https://wintelguy.com/windows-per-vm-licensing-calc.pl)), but where I'm getting hung up / confused is CALs, for Windows, and potentially for MSSQL.

Without getting too detailed, the software we are considering purchasing is designed to help run the billing and "back-end" authentication for the ISP. It (supposedly) includes its *own* RADIUS server (AFAIK, it does **not** use the Windows/Microsoft RADIUS/RRAS subsystem).

The RADIUS server (again, supposedly the Vendor's software, just running on top of Windows), needs to talk to MS SQL to get the "NAS" client information for authentication/authorization/etc. But the customers themselves are generally not going to talk to MS SQL directly. Their equipment (or potentially a router in our infrastructure) would act as a RADIUS "NAS" and communicate with the centralized Vendor RADIUS software, which would then interact with MSSQL.

Does every customer/client piece of hardware that needs to talk to the Windows infrastructure **at all**, require a device CAL? Or would it only need a device CAL if it's actually authenticating/interacting with a *Microsoft service* running on that infra? Is this scenario where I am generally expected to use a Windows "External Connector" CAL, since the access is not going to be performed by an employee of the company?

And before anybody tells me "ask your VAR," that is what I'd love to do, if the company had a VAR setup that could help me with this... long story, and above my pay grade. I wouldn't be asking here if I had someone else I could ask.