Keith, many thanks for your reply. In the meantime I have studied some Ruckus publications and found the solution.
In the repeater mode there is no DHCP server. This is obvious because there are no clients. The MM2211 should be addressed wiith a fixed IP address in the same subset. E.g. 192.168.30.10 if the default IP address of the unit is 192.168.30.1.
This solved my problem. I could address the unit again.