Ethernet connection intermittent Box 12b, drops every couple of minutes

Hi, apologies for English I am still learning Dutch

I have recently purchased KPN and it was activated. I have DSL as glass is not available yet. I have good WiFi connection at all times but my Ethernet connection is dropping every 2 minutes and taking around 30 seconds to reconnect. I have checked the wire with a tester and it is working, I have reset the Box12b.

I am hopeful someone will have had this problem already and know a fix.

Thank you in advance for the help!