From the WST website:

The WST Ethernet Tag Manager may not work with AVM Fritz! Box or Belkin Surf F7D2401 (or other cable modems/routers) Several customers in Europe reported incompatibility issue with AVM Fritz! Box. Belkin Surf F7D2401 is also known to have issues. Some customers have also reported incompatibility with a model of Linksys router. This is often caused by these routers not properly having backward compatibility with 10Base-T Ethernet standard. Solution: Use an Ethernet switch or router (that supports at least both 10Base-T and 100Base-T) between these routers and your Tag Manager.