I tried all versions of TKIP/AES, but it just won't connect in WDS mode, but as you say in your own, it works in "universal repeater" mode.

The strange thing on the 2750, whereas on the 2830, the MAC addresses of the LAN & WIFI were just one digit different apart, on the 2750, the LAN is 00:50:XX:XX:3C:F8 and the WIFI is 5C:33:XX:XX:28:3C.