Btw, I think he is not asking about e-certificates, but CAL that license client access to the system.
IMO, if that TSPlus system is their own implementation of RDP server then no TS/RDS CAL should be required. However you should ask at Microsoft Licensing for authoritative answer.
Also note that as of now, although Microsoft had published the protocol itself as OpenSpec, she also explicitly requires third-party implementations to license the relevant RDP patents. Although the execution of patent right usually don't directly affect customers, the "cease and desist" request may make the company unable to offer further update/support for the product. I suggest you to check with them and see if the product had properly acquired grant on patent usage from Microsoft.