Since C and B are not on the same network segment, we can only use A, which is the secondary proxy you mentioned. Since you have implemented A proxy for B, you can set it up in the same way
Client<===>C<===>A<===>B
I wrote it down briefly:
nginx for C

upstream A{
    server 10.0.0.1:80;
}
server {
        listen       80;
        server_name  www.xxxx.com;

        location / {
                proxy_pass  http://A;
        }
}

A’s nginx

upstream B{
    server 192.168.0.2:80;
}
server {
        listen       80;
        location / {
                proxy_pass  http://B;
        }
}

That’s probably it, but I think you may have other needs