configuring Synology RT2600ac with Netgear CM600

I'm trying to configure Xfinity through Netgear CM600 and then via ethernet cable to Synology RT2600ac. This setup was working, but I had a problem that was outside my house and now I can't get it up and running again after that was fixed. Unfortunately I didn't record the previous configuration, but I know it used to work.

When I connect the ethernet cable from the cm600 to my computer, I can access the internet and its status web page shows no problems.

Then I connect to the ethernet cable to the rt2600 WAN1 port and push the reset button until it reboots. I connect to the Synology wifi and configure it via the web interface. For the operation mode, I use 'Wireless Router' with external access to SRM disabled. The internet connection is 'Auto IP'.

After this setup I log in to the new network and login in again to the rt2600 web interface it says 'failed to set up the internet connection'. In the Status -> Network Center web page, it says internet connection IPv4 Disconnected and IPv6 Limited Connection and it shows values for the IP Address and Gateway. All the relevant lights on the cm600 and rt2600 are green.

What am I doing wrong, and/or how can I debug this?