Flat isnt working for certain Apps, City WLAN isnt working

I reacently discoverd NextDNS and tried it out on my Android smartphone. But i have a few Problems. When im using NextDNS for example Apps like Paypal the App says im am not connected to the Internet. Also i am not able to Connecticut to the WiFi of my City because "the WiFi cannot access the privat DNS Server. Also i cannot access the Help.nextdns.io Website while using the privat NextDNS DNS wich confuses me

Can anyone help me?