Hm, so I don’t know exactly what you mean….
Which Quickline products are you using? Private or business customer?
In the linked document it is about the telephony services and what needs to be configured on the firewall.
But now you mention that you have changed the DNS entries on the router…
What does your installation look like (modem [bridge mode yes/no?], firewall, router, etc.)?
And you can then use the mentioned apps and websites only in the mobile network and not in the WLAN?
What kind of error is displayed or what happens after starting the app or when entering a web page in the browser?
Are any filters / blockers installed and running?