Welcome to the team of RB751 owners 
- It’s not my first MikroTik device, but the only which I can’t deal with.
Here are few links for you:
Boosting RB751U-2HnD Antennas! http://forum.mikrotik.com/t/boosting-rb751u-2hnd-antennas/53678/1
RB751U-2HnD - poor wireless performance & problems http://forum.mikrotik.com/t/rb751u-2hnd-poor-wireless-performance-problems/55441/1
RB751 CPU usage get too high http://forum.mikrotik.com/t/rb751-cpu-usage-get-too-high/54092/1
RB751u-2HnD wireless connection issues with Apple Macbook http://forum.mikrotik.com/t/rb751u-2hnd-wireless-connection-issues-with-apple-macbook/55645/1
There are many more. You can search for them.
Please post output of command: /interface wireless export compact