Jira is already running on Ec2 Linux server and we have set a static IP also. Company TLD is hosted on Route53.
Then on your linux server, ensure you've installed Apache, you can do so by running
sudo su
apt-get install apache2 -y
You should then add a virtual host to the apache default conf file and nano into /etc/apache2/sites-available/000-default.conf to add the below configuration
<VirtualHost *:80>
ServerName sub.example.com
ProxyRequests Off
ProxyVia Off
<Proxy *>
Require all granted
</Proxy>
ProxyPass / http://localhost:8080/
ProxyPassReverse / http://localhost:8080/
</VirtualHost>
ServerName should include the subdomain you want to use from your domain.
Other steps would involve you changing and uncommenting the HTTP or HTTPS Apache proxy Connector (depending if you want to use SSL or not) in your /opt/atlassian/jira/conf/server.xml - given that you used the default installation directory.
After that you can change the CNAME in your domain on route53.
P/S: Also I should have asked if its ubuntu or another linux version as the above is for ubuntu with apache. if you're using Amazon linux with apache installed, you should nano into /etc/httpd/conf/httpd.conf instead and set the virtual host there.