The easiest way is via https://incontrol.ascend.de In rare cases, however, the router is displayed as offline although it should actually be online. In these cases, the following can be checked/tried:
Check whether the connection to the Fusionhub still exists.
If necessary, set up a port forward here and use it to access the router's web interface
Connect to the Fusionhub via VPN and from there to the router's web interface
If the router has a public WAN IP, access can be attempted via this
Access to the LAN IP via a connected PC