AVM FritzBox routers have a special firewall rule for NetBIOS traffic (even though NetBIOS is no longer the correct term) on ports 139 and 445. Only port 445 is used nowadays and especially when connecting to a target by IP or DNS host name.

The rationale is to protect unwitting users from inadvertently accessing file shares on the internet, potentially subverting regular security measures.

On DSL FritzBox routers you can disable this firewall rule. On cable FritzBox routers, you cannot.

The relevant KB article 835 is available in German and other languages, including English. (Make sure to select the correct router model.)

To deactivate the filter, the guide says this:

If it is not possible to access the shared files and printers without NetBIOS and the internet service provider does not block NetBIOS, you can disable the NetBIOS filter in the FRITZ!Box instead:

1. Click "Internet" in the FRITZ!Box user interface.
2. Click "Filters" in the "Internet" menu.
3. Click on the "Lists" tab.
4. Click "Global Filter Settings" to display all of the filter settings.
5. Disable the option "NetBIOS filter enabled".
6. Click "Apply" to save the settings.

There’s a warning about increased attach surface in the guide. It is, however, incorrect or misleading. Disabling this filter does not mean inbound connections are suddenly allowed.