Now, the code I'm going to post works flawlessly in MT4 and the platform is set to x86 in visual studio. But in MT5 (x64) and x64 set as the platform in Visual Studio, I get "Can not find <function name> inside <dll name>.dll".
Never mind. Fixed it.
Re-did DLL from scratch. In the process this time, I set the platform to x64 BEFORE I installed the nugget packages for Robert's DLLExport package.
I should've remembered that.