Most probably an issue with the ads-routes. ADS-Routes have to be configured vise-versa. So CX2020 has to point to the Debian system and the Debian-system to the CX2020.

Check also if there are doubled entries which point to the same ams net id but different ip or something like that. Maybe you get a connection in this case but it will be very instable.

Of course check also the firewall TCP:48898 has to be open on both systems in incoming direction.

You can also check ads-traffic and commands with the ADS-Monitor: https://www.beckhoff.com/de-de/produkte/automation/twincat/tfxxxx-twincat-3-functions/tf6xxx-connectivity/tf6010.html?