To be clear: do the lockups only happen when the Surf SOHO is online via Wifi-as-WAN or does it also lockup when fed by an Ethernet based ISP?
If it locks up again, try getting in via Ethernet from a computer with a hard coded IP address in a valid subnet, no DHCP needed. If this fails, it helps diagnose the problem. If it succeeds, even moreso. While connected, note the state of both the computer and the router ethernet ports LED lights.
And, is there anything of interest in the Event log from just before and just after the lockup?