Yes I tried using hostname of my vm ,but it gave me error as 404

Not Found

The requested URL was not found on this server.                                                        Apache/2.4.41 (Ubuntu) Server at 34.147.128.126 Port 80

Please provide me step by step procedure to solve this issue, I even tried disbaling sudo a2dismod security2 , but it did n't work , I am trying to acess external ip then it is forwarding to Nagiosxi welcome page but after that It is giving me inavlid host header after clicking on acess

For the level of support with a step by step basis it may be best for you to open up a ticket with support. You can do so here.
https://support.nagios.com/