Maybe this is a power savings mode issue I had the same with old Sitecom ones and a Siemens SX-551.

For this connection to your router you have to disable 'power saving mode'. This will drain batteries quicker but it works.

Click on the Network icon on the N8x0 home screen top right (when you are not connected to the failing router) , select 'Connections' -> your router -> next -> next -> until you see 'Advanced' in the very right of the popup, then the 'Other' tab and set power saving mode to Off.