Redmine

Hi there...

In advance I apologize for my English.

I have an account on the domain conditionally "source" with Redmine installed v.3.4.2.
There I have a project with many tasks.
I do not have direct access to the SQL database. I am not the owner of the domain.
The domain administrator "source" has not been contacted for a long time, i.e. I can not get the database dump.
Now I would like to transfer the project to another domain "target", the owner of which is already me. I installed Redmine v3.4.5.
What are the ways to transfer all data from "source" to "target", including tasks, subtasks, comments to tasks, attachemt files, etc.?

I tried this sequence:
1. On the "source" domain - export all tasks to CSV.
2. On the domain "target" - import tasks from the CSV.

The export / import function is implemented crookedly:
- imported tasks are loaded either with errors, or a lot of data is lost, because they simply do not exist in the CSV resulting from the export;
- do not carry over the comments of performers to tasks;
- attachemt files are not migrated;
- etc.

I also found the possibility of export via Atom. The resulting XML file at first glance contains all the necessary data, but there is no possibility
execute its import on the domain "target".

I made a search for a suitable plugin for these purposes, but did not find a full one. All very old and also with limited functionality.

I ask for help with the transfer of the project.