Hello everybody - I am new to the Sophos community and I could not find a solution to my problem in the existing posts. That might be an indication that I might be missing on something very basic.
In our office we have a SG135. Two SD-RED 20 devices are coupled to the UTM. 
The reds1 device is configured in Standard/Split and it works fine - a Windows Box behind the RED gets an IP from the Domain-Server behind the UTM.
The reds2 is planned to be set up the same way but when I configure it identical to red1 and therefore in Standard/Split mode, the windows box behind the reds2 does not get an IP from the Domain-Server but indicates "Unknown network - no internet access". When the reds is configured to Transparent/Split (as shown above) it has internet access but not any access to the Network behind the UTM (and certainly no IP from the internal network).
I configured the interface to bridge both reds2 to the internal network:
I further switched DHCP-Relay to the internal DHCP server. 
The windows box behind reds2 is set to fetch its IP via DHCP.
I am at a loss at understanding why the Windows boxes behind the REDs behave so differently. I should mention that the routers through which the REDs connect are different brands but the REDs both show a stable tunnel to the UTM.
A first analysis of the logs did not show anything special - however, I might be looking up the wrong logs.
I would be thankful for any suggestions.
This thread was automatically locked due to age.