Here are the steps on how to change the DNS on Windows:
Open network preferences by clicking on the WiFi icon on the top of the screen and click on 'Open Network Preferences'
Click on 'Advanced'
Click on DNS
Take a screenshot of this page for reference
Double click on the top item in the list
Change it to 188.8.131.52
Apply changes and try to open the BackOffice again
Please do try on your phone as well to see if it's accessible.