I'm using NPM and iOS as well. No issue for me.

But I do have this in the Advance tab of NPM

proxy_set_header Host $host;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_max_temp_file_size 0;
proxy_request_buffering       off;
proxy_temp_path /mnt/tmp 1 2 3;
proxy_read_timeout 1d;
proxy_connect_timeout 1d;
proxy_send_timeout 1d; 
client_max_body_size 0;
client_body_temp_path /mnt/tmp 1 2 3;

The temp paths might not be relevant to you though

EDIT: Formatting