Tornado server by the side of nginx (in docker)

Question

I recently set up Seatable Enterprise on a VPS. Seatable is delivered as a Docker image that contains nginx. I used Let's Encrypt to generate the key/cert and configure an SSL access. I'm now trying to run a python Tornado server by the side of the web server, but I don't manage to access my webpage.... Here is a minimal version of the Tornado's server.py file to see the configuration (the only notable thing is the port : 8443, and the mapping)

#! /usr/bin/python
import os.path
import tornado.httpserver
import tornado.websocket
import tornado.ioloop
import tornado.web
import asyncio
from tornado_request_mapping import request_mapping, Route

####### Serveur Tornado #######
PORT = 8443
settings = dict(
    template_path = os.path.join(os.path.dirname(__file__), "templates"),
    static_path = os.path.join(os.path.dirname(__file__), "static")
    )    

@request_mapping("/products")
class PainMainHandler(tornado.web.RequestHandler):
    @request_mapping("/order.html")
    def get(self):
        print("[HTTP](MainHandler) User Connected.")
        self.render("products/order.html")

@request_mapping("/ws")
class PainWSHandler(tornado.websocket.WebSocketHandler):
    def open(self):
        print('[WS] Connection was opened.')
 
    async def on_message(self, message):
        print('[WS] Incoming message:', message)

    def on_close(self):
        print('[WS] Connection was closed.')

if __name__ == "__main__":
    try:
        application = tornado.web.Application(**settings)
        route = Route(application)
        route.register(PainMainHandler)
        route.register(PainWSHandler)
        http_server = tornado.httpserver.HTTPServer(application)
        http_server.listen(PORT)
        main_loop = tornado.ioloop.IOLoop.instance()

        print("Tornado Server started")
        main_loop.start()
    
    except:
        print("Exception triggered - Tornado Server stopped.")

Trying this locally, everything works fine (I see the content of my order.html page using the address http://localhost:8443/products/order.html).

I uploaded these files on my VPS to run the tornado's webserver by the side of the nginx server but whatever I tried, I'm still getting an error (400 or 414 or 512 or the custom 404 page from Seatable). Here is a part of the nginx.conf file:

server {
    if ($host = myserver.vps.ovh.net) {
        return 301 https://$host$request_uri;
    }
    listen 80;
    server_name vps-25355c52.vps.ovh.net;
    return 404;
}

server {
    server_name myserver.vps.ovh.net;
    listen 443 ssl;

    ssl_certificate /shared/ssl/fullchain.pem;
    ssl_certificate_key /shared/ssl/privkey.pem;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2 TLSv1.3;
    ssl_ciphers HIGH:!aNULL:!MD5;

    # for letsencrypt
    location /.well-known/acme-challenge/ {
        alias /var/www/challenges/;
        try_files $uri =404;
    }

    proxy_set_header X-Forwarded-For $remote_addr;

    location / {
        add_header Access-Control-Allow-Origin *;
        add_header Access-Control-Allow-Methods GET,POST,PUT,DELETE,OPTIONS;
        add_header Access-Control-Allow-Headers "deviceType,token, authorization, content-type";
        if ($request_method = 'OPTIONS') {
            add_header Access-Control-Allow-Origin *;
            add_header Access-Control-Allow-Methods GET,POST,PUT,DELETE,OPTIONS;
            add_header Access-Control-Allow-Headers "deviceType,token, authorization, content-type";
            return 204;
        }
        proxy_pass         http://127.0.0.1:8000;
        proxy_set_header   Host $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-Host $server_name;
        proxy_read_timeout  1200s;

        # used for view/edit office file via Office Online Server
        client_max_body_size 0;

        access_log      /opt/nginx-logs/dtable-web.access.log;
        error_log       /opt/nginx-logs/dtable-web.error.log;
    }

I tried adding a new server configuration like

server {
        listen 8443;
        server_name myserver.vps.ovh.net;
        location /pain {
                proxy_pass      http://127.0.0.1:8443/products/order.html;
                proxy_redirect  off;
                access_log      /opt/nginx-logs/pain.access.log;
                error_log       /opt/nginx-logs/pain.error.log;
        }
}

with proxy_pass http://127.0.0.1:8443/products/order.html; or proxy_pass http://127.0.0.1:8443/; I also tried to add a new location /products in the second server configuration, with the same two proxy_pass variants as before, but nothing worked. I'm sure I miss something, do you have any idea ?

Thanks a lot