Accessing .net core website hosted on Ubuntu 16.04 through internet - linux

Question

I am following along this tutorial to deploy my asp.net application onto Ubuntu. I have installed .Net core 2.0 on my Ubuntu machine using this post. NO issues with that excellent article.
My VM configuration is :
user#instance-2:~/dev/mywebsite$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.3 LTS
Release: 16.04
Codename: xenial
I have confirm that .Net core is installed properly.
user#instance-2:~/dev/mywebsite$ dotnet --info
.NET Command Line Tools (2.0.0)
Product Information:
Version: 2.0.0
Commit SHA-1 hash: cdcd1928c9
Runtime Environment:
OS Name: ubuntu
OS Version: 16.04
OS Platform: Linux
RID: ubuntu.16.04-x64
Base Path: /usr/share/dotnet/sdk/2.0.0/
Microsoft .NET Core Shared Framework Host
Version : 2.0.0
Build : e8b8861ac7faf042c87a5c2f9f2d04c98b69f28d
Next I have created new dot net web application using below command
dotnet new web
set ASPNETCORE_URLS environment variable
export ASPNETCORE_URLS=http://+:5123
Confirmed that its set properly.
user#instance-2:~/dev/mywebsite$ echo $ASPNETCORE_URLS
http://+:5123
Ran my application using dotnet run. Can see below message on console.
user#instance-2:~/dev/mywebsite$ dotnet run
Hosting environment: Production
Content root path: /home/hemant_shelar/dev/mywebsite
Now listening on: http://[::]:5123
Application started. Press Ctrl+C to shut down.
Expected: I should be able to browse this site from internet using IP address
like : http://35.185.21.120:5123/ But I am not able to access this site from outside. Getting ERR_CONNECTION_TIMED_OUT error.
Note: 35.185.21.120 is IP address of my Ubuntu machine which is in google cloud.
Am I missing any configuration?
UPDATE 1:
I have open port 5123 using below command
sudo ufw enable
sudo ufw allow 5123
Verified that port is opened using below command:
hemant_shelar#instance-2:~/dev/mywebsite$ sudo ufw status
Status: active
To Action From
-- ------ ----
5123 ALLOW Anywhere
5123 (v6) ALLOW Anywhere (v6)
But still same issue.
UPDATE 2
I set environment variable ASPNETCORE_URLS to http://+:80
hemant_shelar#instance-2:~/dev/mywebsite$ echo $ASPNETCORE_URLS
http://+:80
hemant_shelar#instance-2:~/dev/mywebsite$ sudo dotnet run
Hosting environment: Production
Content root path: /home/hemant_shelar/dev/mywebsite
Now listening on: http://localhost:5000
Application started. Press Ctrl+C to shut down.
But when I run website its listing on port 5000. Is there any other way to expose site on port 80? e.g. in IIS we use http host header to open multiple sites on port 80.