Nginx Caching Content Config

Question

I have to serve .mp4 files through Nginx on a portable Jetson machine. .mp4 files are fragmented MP4 that grow over time (recordings). I would like to be able to serve them with byte-range download and optimal caching (I found that slice-by-slice might be a good idea (https://www.nginx.com/blog/smart-efficient-byte-range-caching-nginx/#cache-slice).

At the moment, my config (/etc/nginx/sites-enabled/default) looks as follows:

proxy_cache_path /tmp/my_cache keys_zone=my_cache:10m;

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    root /;

    index index.html index.htm index.nginx-debian.html;

    server_name _;

    location /api {
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_redirect off;
        proxy_pass http://localhost:4000;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";
        client_max_body_size 100M;
    }

    location ~ ^/external/(.*)$ {
        proxy_force_ranges on;
        proxy_cache my_cache;
        proxy_ignore_headers Set-Cookie;
        proxy_hide_header Set-Cookie;
                
        slice 1m;
        proxy_cache_key $host$uri$slice_range;
        proxy_set_header Range $slice_range;
        proxy_http_version 1.1;
        proxy_cache_valid 10m;
        proxy_cache_min_uses 1;
        proxy_pass http://127.0.0.1;
                
        root /media/nvidia;
        try_files /$1 =404;
    }

    location / {
        proxy_force_ranges on;
        proxy_cache my_cache;
        proxy_ignore_headers Set-Cookie;
        proxy_hide_header Set-Cookie;
                
        slice 1m;
        proxy_cache_key $host$uri$slice_range;
        proxy_set_header Range $slice_range;
        proxy_http_version 1.1;
        proxy_cache_valid 10m;
        proxy_cache_min_uses 1;
        proxy_pass http://127.0.0.1;
                        
        root /var/www/html;
        try_files $uri $uri/ =404;
    }
}

The problem is that after requesting file chunks, there is nothing coming up in the /tmp/my_cache folder, and the requests similar amount of time. What am I doing wrong?

Thanks