So well, one connectivity problem is solved: the connection to WiFi is established.
The internal antenna of Model D does not allow to be more than few meters away from the access point.
Now the device is placed around 1meter away from the access point.

But still, the upload of the measured data is often stopping through the day.
Whilst the unit indicates it is connected to the hotspot, the webpage says the device is offline.

Through the debug menu it is possible to re-establish the connection.
Now, sometimes it is possible to upload the offline data, sometimes – even if for few hours there was no data update,
the SYNC menu says there is no offline data available for upload.
Yesterday I did upload over 3000 datasets but I could not see this reflected on the webpage.
It would be good if the webpage would mark those data, which has been uploaded through SYNC menu.
In this way users could become an indication, that the SYNCed data is not lost.
this is especially important due to the fact, that the device erases the offline data after the upload.
Maybe it would be better to let erase it first after the webpage confirms the reception.

Greets
Damian