Thanks for the hints. I did use the correct branch which includes the downlink code, so this cannot be the root cause.
Today i tried to delete the gateway on TTN and created a new gateway with the same EIU. My expectation was that TTN would “forget” the attributes of the gateway, but it didn’t. After the new gateway with same EIU was created, the old counter for received packets was displayed in console. So i think the TTN backend remembered all the attributes of the old gateway. Still no txpk on port 1700 