Airbnb guests could not use the app

I recently bought and installed my Nuki Smart Lock and Bridge. Getting it to work cost my 12+ hours… not a very good start.
I order to get the Nuki Smart Lock installed on my front-door I had to replace my old knob-cylinder with a new one because my knob cylinder was not supported.

Two days later, my first guest to use to lock arrived. They had received the Nuki email with the links to download the Nuki app and to receive their personal activation keys, but they had not installed the app yet. Fortunately, I was at home so entering the house was no problem. But before leaving them I wanted them to install the App and make sure they could operate the lock.

Installing the App, of course, was no problem. But getting the app to work …

We tried for about 10 minutes but their Nuki apps, although they were connected to my home wifi network, showed that they were off-line.

Not wanting to leave them without the use of the front-door, and not having enough keys for my new cylinder, I decided to once again replace the new cylinder with the old knob cylinder.

Does the app need some sort of initialization time? Do my guest need to install the app, long before their arrival? Is there a synchronization issue between the app and the server? Or should everything work as soon as it is installed?