I used to work fine with my old Ziggo modem, together with 3 Deco M9 devices set up as an access point.

Now I had to change my old Ziggo Modem to  a new Ziggo ConnectBox, so I thought it would be simple: just connect the cable from the old modem/router port to a port on the new one. That worked well for about an hour, but now I only have 0,5Mbit download or no internet at all via the Deco. (should be 300Mbit according to a speed test done on a pc directly connected to the Ziggo connectbox)

I already did a reset on all devices, reconfigured it, tried the router DHCP server option etc... but the issue remains...

What can I do?

Thanks!