Docker swarm with TOMCAT and NGINX as load balancer

I am trying to set up a docker swarm where NGINX is used for the load balance of the swarm nodes.

docker swarm init --advertise-addr 192.168.99.153

Also, I add the two nodes as a swarm

docker swarm join --token SWMTKN-99-31eka5srtjyvajsbiusixh6gil8p6xuk6yejfts4co8voxu8op-a8awcomh58de8d0ofkckqnsud 192.168.99.153:2377

Now I create a service on the manager node using

docker service create --name backend --replicas 2  --publish 8080:80 tomcat

I tested the tomcat on node1/node2 browser by 192.168.99.153:8080 / 192.168.99.154:8080 and I can see tomcat landing page.

Now I set up a separate swarm on 3rd node for Loadbalancer NGINX.

docker swarm init --advertise-addr 192.168.99.156

after this create the Nginx conf file

sudo mkdir -p /data/loadbalancer
sudo vi /data/loadbalancer/default.conf

and add following lines in this file

  server {
       listen 80;
       location / {
          proxy_pass http://backend;
       }
    }
    upstream backend {
       server 192.168.99.153:8080;
       server 192.168.99.154:8080;
    }

after this when I create an NGINX service with

docker service create --name loadbalancer --mount type=bind,source=/data/loadbalancer,target=/etc/nginx/conf.d --publish 80:80  nginx

When I launch the 192.168.99.156:80 on the browser, I see the 502 Bad Gateway error page.

Question:-

1. Some time tomcat server is also not accessible after creating the backend service.

2. Why tomcat is not coming up using NGINX load balancer?