The original code is using the legacy SSL Com devkit - which does not support RSSL/OMM
You will need to rewrite the code using a more recent API/Devkit
Your choices will depend on your programming language restrictions/choice.
Recommend APIs are the ElektronSDK or the Websocket API - details of both can be found here
If you are able to use Java or C++ then the Elektron Message API from the ElektronSDK would be your best choice.
If however, you cannot use Java or C++, then you can use the Websocket API - which is language agnostic.
Furthermore, If you are prepared to use an EAP/beta version of a library then you could also consider the Refinitiv Data Platform Library - which is available in .NET and Python - and is probably the easiest to use.
More info on RDP Library
RFA.NET does not use the DLLs.
It seems to be SFC /SFC COM DLLs. Are you moving from SFC to RFA.NET?
RFA.NET is a .NET wrapper class built on top of RFA C++. It uses C++/CLI to implement the library, so it has an interop between unmanaged code(C++) and .NET layer. Also, RFA.NET has a different architecture and implementation comparing with SFC. So you have to re-write the application, and you are unable to use your old implementation.