If you still get a trial message this means that the app is still using the trial Telerik.Web.UI.dll assembly.
In order to remove the error, please follow the instructions in this article: Upgrade From Trial to Licensed Version of Telerik® UI for ASP.NET AJAX, e.g.
Download the Telerik_UI_for_ASP.NET_AJAX_2019_1_215_Dev zip installation from https://www.telerik.com/account/product-download?product=RCAJAX and replace the trial assembly in the bin folder with the registered one from the dev zip. Update the reference to it.
Please try it and let me know how it goes.
Get quickly onboarded and successful
with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers. Learn More.