Hi godofal,
I don’t have instructions for your specific setup, but my first search result for “nextcloud max upload size” got this thread on Reddit from three years back
Since this person also wants to upload files of 20 GB, this story is such a good match that I am sure you already have seen that thread and must look for another solution. On the other hand, you did not mention which specific steps you already have undertaken, so it may help.
For future visitors I’ll post a copy here in case Reddits management becomes more awful than it is and the source is not available anymore:
Thanks to u/iams3cond i could make it work:
php → php-local.ini
upload_max_filesize = 20G post_max_size = 21Gnginx->site-confs->default
client_max_body_size 20G