Hello  @Tmrchtr ,

Welcome to our community.

According to Google Translate, the LAN port on RE255BE is not working. If there is any discrepancy, please describe it in English.

Here are some troubleshooting steps that you can try:

1. First, confirm that the extender's internet is working by connecting a device to its Wi-Fi network.
2. Check whether the Ethernet light on the extender is on. If it's not, try a different Ethernet cable.
3. If possible, connect your NAS directly to one of the LAN ports on your FritzBox to verify the NAS itself is working fine.
4. If the NAS works fine with the router, power cycle the extender and check again.
5. Hard reset the extender by using a pin to press the reset hole for one second, then reconfigure it.

If the extender appears to be connected to your FritzBox but has no internet, try the following:
1. Rename the extender's Wi-Fi network (SSID) to something different from your FritzBox's Wi-Fi name, as having the same name can cause conflicts. (Guide: How to Change Your TP-Link Range Extender Wi-Fi Password or Network Using Tether)
2. Upgrade the extender's firmware to the latest version. (Guide: How to Update Your TP-Link Extender Firmware: Three Methods)
3. Compare the IP address obtained from the router and the extender. If they are not in the same network segment, enable the DHCP server on the extender. (Guide: TP-Link Range Extender Not Connecting to Router: Troubleshooting Guide)

Also, make sure your FritzBox does not have MAC filtering or Access Control enabled, as this could block the extender from accessing the internet.