BTW, all of this is totally off-topic and not related to Nextcloud. You should ask aunt G. for further details to understand how DNS on a Fritzbox works in detail.
All DNS requests coming from the LAN are send to the Fritzbox, because the DHCP server on the router provides it to all devices on your LAN. The Fritzbox isn’t a fully blown DNS server and can only resolve internal names, like fritz.box or the short names of your local resources. Therefore it forwards all requests which cannot be resolved locally, to the configured external DNS servers on the Internet. The reply from the DNS servers on the internet are then returned to the client.
As a result it knows every resource on your LAN which have a local IP assigned, like 192.168.178.x and additionally the external address of servers on the Internet.
If you now send a DNS request for your external DNS name to the Fritzbox it is forwarded to the internet and then resolved to the external IP address. If you now try to reach this server all packets are routed over the Fritzbox to the internet and cannot be routed back to your LAN. AVM explains all this in detail in their support forum:
I would personally recommend to play with the rebind function, which might solve the problem (I personally never used it), use a different host name or the ip address on your LAN or install your own local DNS where you can overwrite DNS requests from your LAN and assign a local ip address.