Short answer is that you can’t get a minidump symbolicated to C# because C# compiles to IL and what you’ll get in the minidumps is JIT’ed code. Unless it was IL2CPP which means you could likely get this symbolicated to the C++ code generated from the .NET IL.
Are you using IL2CPP or is this really Mono with JIT? and there’s a native crash there that brings the process down.