Hello, tell me, please at the expense mikrotika 751u-2hnd.
Connect an external omni-directional antenna TQJ-2400-12 dB, and the difference in the signal amplification is not that the antenna, that without it, the same signal level.
What could it be?

If you want to activate external antenna, Open tab HT on wireless interface properties, then set HT Chain:
- set chain 0 uncheck, and check chain 1 for tx and rx.
- set antenna mode = antenna-b

Good Luck.

we recommend to leave RX for chain0 enabled so it would work correctly.