RE450 (and other extenders) have a problem with the Mikrotik DHCP server. http://forum.mikrotik.com/t/ros-6-38-serious-dhcp-server-problem/105259/1
A lot of tests have been done. "The Mikrotik DHCP offer does not reach the client device behind the repeater. "
The only known workaround is to enable the DHCP server on the RE450, with the same subnet as the main AP, but a different pool. (AP handles some addresses, RE450 handles other addresses)
Once the IP address is assigned, everything else will work. Of course using a fixed IP address could also be a workaround.
After loading OpenWRT on the RE450, the problem remains the same. The “relayd” in OpenWRT couldn’t solve it for the Mikrotik DHCP (in my tests).
https://forum.openwrt.org/t/re450-as-a-repeater/693
(Installed OpenWRT because the RE450 is using some uncontrolled channels in ‘turbo’ mode (different band out))