From memory I think the value you pass on the second param of the MapRoutes need to match the Url in your GuiPlugin declaration.
For example you see in this post https://blog.novacare.no/creating-episerver-admin-plugins-using-mvc/ both the URL and the Route param match. Also the URL isn't a path to a file it's just a unique path to identify your module. So I would change both values to "custom-plugins/securitytxt"
One other point, this way of creating plugins has been removed for the .NET 5 version (CMS 12) so if you have any expectation of upgrading in the future I'd create it using a menu item in the editor as show here https://world.optimizely.com/blogs/mark-stott/dates/2021/8/extending-the-admin-interface-in-optimizely-cms-12/ which works for now and for CMS 12 (.NET 5)
Thanks for the help Scott, I managed to get the URL right in the end, now it's pointing to the right controller and action. Evidently the blog post (which I have read a few times) was a big help, and there's a small "/" at one of the url-pointers that I missed. Working fine now!
I've searched far and wide across the web for a solution, but being completely new to Episerver and the world of CMS, I just can't seem to get it right.
I manage to get my plugin to show up in the admin tools menu, and up until yesterday it actually crashed visually for me. Now I didn't get anything until I actually opened the console and it seems to return "the server responded with a status of 404 ()". Any ideas how to fix this?
Here's my controller that I try to get to fire:
And my initialization: