If you are using our web ReportDesigner or ReportViewer as an embedded control in your application, then you have add the custom assemblies in that application. Also, please include the version and public key details for the custom assemblies as shown in the below example in report.
Reports_CustomLibrary, Version=22.214.171.124, Culture=neutral, PublicKeyToken=null
Reports_CustomLibrary, Version=126.96.36.199, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51
If you still face any issue then please share your RDL report with us, it will be helpful for us to validate the issue at our end.