What you want to obtain is seemingly a switch (no routing, no nat, all ports into a bridge) with a wireless connection to the rest of the wifi network (station or station-pseudobridge mode, i.e. “client”).
When you have wlan1 as self-standing, the DHCP client should be running on it.
The moment you add wlan1 to the bridge, wlan1 “loses some dignity” as it is now part of the bridge, so that setting is not valid anymore, and the DHCP client should be running on the outer envelope (the bridge).
If you are running 7.x, a basic configuration should look loosely like this one:
http://forum.mikrotik.com/t/hap-ax-lite-lte6-how-to-set-as-repeater/180534/1
Then it is your choice to add or not a slave wlan interface (or use the second radio) to have the device act also as repeater/extender.