It's clear that you encounter problems with your connectivity.
Athough I can't be sure without some tests, I guess that -when in trouble- you can reach IPv6-IPv4 Dual Stack Sites like www.google.com and not IPv4-only sites like www.ziggo.nl. Most Sites are IPv4-only.
Of so then you have problems with your IPv4 connectivity.
Thanks for the picture of the Ipv6 test.
I never saw this from someone that uses IPv6 DS-Lite Firmware, like you.
Browser Fallback = no is in my opinion not a bad sign in your test case. Not really sure about this.
You have an IPv6 Connection with the test Site.
I think the Site tries to test the IPv4 address you seem to have.
But this IPv4 address is not in use for the Connection you made with the test Site. So: no falback detected.
It is an IPv4 address of the CGNAT (ake AFTR) Server your Router is communicating with over an IPv6-only Connection.
(IPv4 traffic is encapsulated in this IPv6 Connection)
This IPv4 address is not an external address of your Router, and is shared with other users of the CGNAT.
This address only comes into operation for you when an IPv4 Connection is necessary from your LAN.
I think that this kind of operation fails when needed, if my guess is right that you can only make IPv6 Connections to the outside world when things go wrong.
This indicates intermittent improper functioning of the CGNAT - DS-Lite combination.
You may possibly solve the problem by a factory reset of the ConnectBox.
It's in Dutch, but you definitely will find your way.