NGINX Reverse-Proxy : Cannot Upload bigfile quickly towards NAS Synlogy DSM7

Question

I can't upload a file larger than 1349 Mo towards my NAS Synology (DSM7) through my NGINX reverse proxy. Sure I set client_max_body_size 10g; or client_max_body_size 0; but the real limit size looks like to be approximately 1,3 Go and the message is : Connection failed. Please check your network settings. Admit that it's ugly to get this message at 98% of the task with a 1.4 GB file ! I tried many settings in nginx.conf as:

sendfile on;
tcp_nopush on;
tcp_nodelay on;
types_hash_max_size 2048;
keepalive_timeout 3600;
server_tokens off;
server_names_hash_bucket_size 64;

or in my proxy conf in sites-available :

proxy_request_buffering off;
proxy_buffering off;
proxy_buffer_size 16k;
proxy_busy_buffers_size 24k;
proxy_buffers 64 4k;

The only ones settings allowing a upload bigger than 1,3 Go and maybe until infinity are :

proxy_request_buffering off; proxy_buffering off;

But the upload speed is divided by ten !!! Unacceptable.

Nor the time and size limit solved my problem, and I didn't find any proxy buffering setting or something like that

I'm really disappointed with NGINX when I never had this kind of problem with apahe, but I'm sure there is a solution I haven't seen. Is Someone able please to solve my problem ?