Unable to port forward Docker Nginx reverse proxy

I've spent a lot of time on this, and it's time to use a lifeline. I'm running the wonderwall/nextcloud Docker container, and I set up an Nginx container to proxy it. It works fine in my internal network, but when I try to port forward through my Ubiquiti USG, every attempt to connect from outside results in a timeout. I know my USG is functioning correctly, because another Docker container I use that is not running Nginx works fine using the same macvlan network as below (physical).

My docker-compose file:

networks:
   backend:
      driver: bridge
   physical:
      external: true
services:
   nextcloud-app:
       container_name: nextcloud-app
       depends_on:
          - nextcloud-db
       environment:
          - UID=1031
          - GID=1031
          - UPLOAD_MAX_SIZE=10G
          - APC_SHM_SIZE=128M
          - OPCACHE_MEM_SIZE=128
          - CRON_PERIOD=15m
          - TZ=(redacted)
          - ADMIN_USER
          - ADMIN_PASSWORD
          - DOMAIN=(redacted)
          - DB_TYPE=mysql
          - DB_NAME=(redacted)
          - DB_USER=(redacted)
          - DB_PASSWORD=(redacted)
          - DB_HOST=nextcloud-db
       image: wonderfall/nextcloud:latest
       networks:
          backend: null
       restart: unless-stopped
       volumes:
          - nextcloud_apps:/apps2
          - nextcloud_config:/config
          - nextcloud_data:/data
          - nextcloud_themes:/nextcloud/themes
   nextcloud-db:
      container_name: nextcloud-db
      environment:
         - MYSQL_ROOT_PASSWORD=(redacted)
         - MYSQL_DATABASE=(redacted)
         - MYSQL_USER=(redacted)
         - MYSQL_PASSWORD=(redacted)
      image: mariadb:10
      networks:
         backend: null
      restart: unless-stopped
      volumes:
         - nextcloud_db:/var/lib/mysql
   nextcloud-web:
      container_name: nextcloud-web
      depends_on:
         - nextcloud-app
      image: nginx
      networks:
         backend: null
         physical:
            ipv4_address: 192.168.1.10
      restart: unless-stopped
      volumes:
         - nextcloud_proxy_config:/etc/nginx/conf.d
         - nextcloud_proxy_ssl:/certs
version: '3.0'
volumes:
   nextcloud_apps:
      external: true
   nextcloud_config:
      external: true
   nextcloud_data:
      external: true
   nextcloud_db:
      external: true
   nextcloud_proxy_config:
      external: true
   nextcloud_proxy_ssl:
      external: true
   nextcloud_themes:
      external: true

My /etc/nginx/conf.d/default.conf file:

server {
   listen 80;
   server_name (redacted);
   return 301 https://$host$request_uri;
}

server {
   listen 443 ssl http2;
   server_name (redacted);

   client_max_body_size 10G;

   ssl_certificate     /certs/server.crt;
   ssl_certificate_key /certs/server.key

   location / {
      proxy_pass http://nextcloud-app:8888;
      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-Remote-Port        $remote_port;
      proxy_set_header        X-Forwarded-Proto    $scheme;
      proxy_redirect          off;
   }
}

Lastly, the configuration of the physical macvlan network:

$ docker network inspect physical
[
    {
        "Name": "physical",
        "Id": "999be40dea19b51fbeb3bda5501288f52365aa2ce155b5a5fedede3270fdf959",
        "Created": "2017-04-12T00:45:37.306899171-07:00",
        "Scope": "local",
        "Driver": "macvlan",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "192.168.1.0/24",
                    "Gateway": "192.168.1.254"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Containers": {
            "92f04d62dfd43c5b9bb6ef290186bd5f7926b82d8232ee1bd6e488694d086240": {
                "Name": "(redacted)",
                "EndpointID": "c547179a88c11681d9abd83f020a92b1a1f341575d12423a7e0e5f3efe1d9a40",
                "MacAddress": "(redacted)",
                "IPv4Address": "192.168.1.9/24",
                "IPv6Address": ""
            },
            "cd14e07bbf7435274610ff10ec2a16035df71440772ab63f8e4973599d809b60": {
                "Name": "nextcloud-web",
                "EndpointID": "57e86f161ea2d578aadb6b99bfabe6da6dcdf2070ff91bfd30c0f450b076bc63",
                "MacAddress": "(redacted)",
                "IPv4Address": "192.168.1.10/24",
                "IPv6Address": ""
            }
        },
        "Options": {
            "parent": "eno1"
        },
        "Labels": {}
    }
]

I'd be very grateful for any insights!