Maintenance page does not work

Hello,
The maintenance plugin does not work: I activated the maintenance page (switch is on and change is saved), preview is working but in different browsers and on different devices (PC, Laptop, Smartphone and Tablet) the original page is active.

Do you have any idea what is wrong? (Cache is already cleared).

Thank you in advance.

Best regards,

Markus

The page I need help with: [log in to see the link]