Hi Hans,
On a remote like a phone or tablet running Android or iOS, when you try and login and cannot connect there will be a screen that shows up and you should see a line which says “Make sure your Roon core is powered on and connected to your network.”
Below that line there is a blue “Help” link. Click that “Help” link and you will have a pop up which allows you to enter the IP of your Roon Core.
That is where you enter
255.255.255.255
This will need to be done on each remote that has a connection issue.