Very quickly I am able to access the contacts page but from the contacts page I have tried to go Dashboard by clicking dashboard, It again takes time to load the dashboard, and sometimes 504 Gateway Time-out as well.
The problem here is your base dashboard is to heavy to load, so it either needs more memory / CPU, or you need to create a very basic dashboard with just one widget on it. Not ideal but you could then look at our post on setting up Metabase and Surge. your dashboard (Surge Mautic Dashboard into something Amazing!).