As covered in How To Install Apache Tomcat 8 on Ubuntu 16.04

By default, newer versions of Tomcat restrict access to the Manager and Host Manager apps to connections coming from the server itself. Since we are installing on a remote machine, you will probably want to remove or alter this restriction. To change the IP address restrictions on these, open the appropriate context.xml files.

If you followed that installation tutorial, those file can be found at:

• /opt/tomcat/webapps/manager/META-INF/context.xml

• sudo nano /opt/tomcat/webapps/host-manager/META-INF/context.xml

To allow remote access, edit the files and comment out the part of the configuration file specifying the IP address restriction or replace them with a list of IP addresses that you would like to whitelist.

<Context antiResourceLocking="false" privileged="true" >
  <!--<Valve className="org.apache.catalina.valves.RemoteAddrValve"
         allow="127\.\d+\.\d+\.\d+|::1|0:0:0:0:0:0:0:1" />-->
</Context>

Restart Tomcat for the changes to take effect:

sudo systemctl restart tomcat

Remember, since Apache mod_jk makes a local connection to Tomcat on port 8080 and proxies it to port 443 over HTTPS (or port 80 if you are not setting up SSL), you will access it at https://your_ip_address/manager/html