Problem in running SSRS report using custom dll in SSRS 2016. Failed to grant permission to execute

When i am running the report, it is supposed to access a custom assembly (which is not signed), But i am getting the below error.

Failed to load expression host assembly. Details: Could not load file or assembly "xxxxxxx, Version=6.0.0.1, Culture=neutral, PublicKeyToken=null" or one of its dependencies. Failed to grant permission to execute. (Exception from HRESULT: 0x80131418) (rsErrorLoadingExprHostAssembly)

I have set FullTrust for the dll in RsSrvpolicy.config file.

The dll was compiled in .net framework 2.0.

OS: Windows 2016 SQL 2016