KDEConnect not working?

[Teunis]

A strange problem, I can no longer get KDEConnect to work.
Because it also doesn't work between Android devices I'm wondering if it has anything to do with the new modem/router I got a few days ago.
One problem is I'm not quite sure when was the last time I saw it work

[Teunis]

In the mean time I found a new check box in the router's WLAN Security set up for:
The active wireless devices displayed below may communicate with each other.

New to me...
(Fritz!box 7360)