Matrix Synapse admin UI behind NGINX reverse proxy

I set up a Matrix Synapse server and everything is working fine so far.

My only problem is accessing the admin UI which I got from github Awesome-Technologies / synapse-admin

I symlinked the index.html into nginx webroot at /var/www/html and wrote another server block in my config as well as customised the config on my reverse proxy on a separate server. I already tried with different ports and location directives but somehow nothing seems to work. Concerning documentation or working examples, information on this project are quite scarce. Am I missing something?

Matrix server conf

server {
    listen 8080;
    server_name matrix.example.tld www.matrix.example.tld;
    location /_matrix {
        proxy_pass http://localhost:8008;
        proxy_set_header X-Forwarded-For $remote_addr;
        client_max_body_size 50M;
    }
}

#Matrix Federation
server {
    listen 8448;
    server_name matrix.example.tld www.matrix.example.tld;

    location / {
        proxy_pass http://localhost:8008;
        proxy_set_header X-Forwarded-For $remote_addr;
    }
}

server {
    listen 80;
    root /var/www/html;
    index index.html index.htm

    server_name _;

    location / {
        try_files $uri $uri/ =404;
    }
}

Reverse Proxy conf

##Matrix
server {
    listen 80;
    server_name matrix.example.tld www. matrix.example.tld;
    return 301 https://$host$request_uri;
}

server {
    listen 443 ssl;
    server_name matrix.example.tld www. matrix.example.tld;

    location /_matrix {
        proxy_pass http://SRV_IP:8080;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_set_header X-Forwarded-Proto $scheme;
        client_max_body_size 50M;
    }

    location /_synapse/admin {
        proxy_pass http://SRV_IP;
        proxy_set_header X-Forwarded-For $remote_addr;
    }

    # Federation Port
    listen  8448 ssl;

    location / {
        proxy_pass http://SRV_IP:8448;
        proxy_set_header X-Forwarded-For $remote_addr;
    }

    ssl_certificate /etc/letsencrypt/live/matrix.example.tld/>
    ssl_certificate_key /etc/letsencrypt/live/matrix.example.tld >
    include /etc/letsencrypt/options-ssl-nginx.conf;
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem;


}