Nginx reverse proxy subfolder with nextcloud

I have a Nextcloud server deployed at address 192.168.6.20 (snap install with ubuntu 20.04 LTS). It's a plain http, so no https. If I point a browser to http://192.168.6.20 it works.

Now I want another server to be its nginx reverse proxy, by pointing at https://frontend.mydomain.com/cloud I would like to access the nextcloud install.

I'm in trouble with the fact I want to access using a subdirectory /cloud on the frontend. In the Nginx configuration I wrote something like:

upstream nextcloud {
server 192.168.6.20:80;
keepalive 32;
}
location ~ ^/cloud/(.*) {
rewrite  ^/cloud/(.*) /$1 break;
client_max_body_size 50M;
proxy_set_header Connection "";
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Frame-Options SAMEORIGIN;
proxy_buffers 256 16k;
proxy_buffer_size 16k;
proxy_read_timeout 600s;
proxy_http_version 1.1;
proxy_pass http://nextcloud;
proxy_redirect off;
}

But all I get is an unformatted white page with some text complaining that I'm not connecting from a trusted ip. Besides the complaint, the page is white with unformatted text, so there's something not working. If, for example, I create another configuration file to let the url be http://frontend.mydomain.com without the path /cloud and I create it as:

upstream nextcloud {
server 192.168.6.20:80;
keepalive 32;
}
location / {
client_max_body_size 50M;
proxy_set_header Connection "";
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Frame-Options SAMEORIGIN;
proxy_buffers 256 16k;
proxy_buffer_size 16k;
proxy_read_timeout 600s;
proxy_http_version 1.1;
proxy_pass http://nextcloud;
proxy_redirect off;
}

it works: I mean, it still complains for the untrusted source, but the page is well formatted, with form texts, blue background and so on.

How can I achieve what I need? What am I missing/doing wrong?

Did you set up trusted proxies in config.php? It's in the manual: https://docs.nextcloud.com/server/23/admin_manual/configuration_server/reverse_proxy_configuration.html