thanks for your reply.
Yes I read the two pages - a Micro-SD card has been shipped with the device and was already prepared with turris version 4.0 .
I also tried to prepare a new one as described on https://doc.turris.cz/doc/en/howto/mox_microsd_card - same result.
MOX has only one ethernet port and the setup is used according to the third pic on https://doc.turris.cz/doc/en/howto/cpu_only
Ethernet cable works without problems with different laptops.
now I have an dns lease for the MOX. Accessing its IP through browser ends in a timeout.
No data, no setup guide.
tcpdump shows http requests from my computer but no response from MOX .
Requesting triggers the red led on the MOX . After some seconds it goes off again.
About 5 - to 10 minutes later the device disconnects from LAN completly.