In order to verify that you have changed the right file and that the settings haven’t been overwritten (by .htaccess file). The default .htaccess of Nextcloud sets it to ~512 MB.
Have you found a solution? I have the same problem.
I have set the upload_max_filesize and post_max_size to 1024M in all config files: .user.ini, .htaccess, php.ini.
But I can upload files with 3GB without an error. In the admin settings under server info the upload limit is shown with 1GB.
I think the reason is that nextcloud was changing upload method from HTTP-POST to HTTP-PUT. Both parameters upload_max_filesize and post_max_size have no effect on PUT.
In this context some filesize checking was removed from code (see github).