in recent months I tested RB751G/RB751U wireless performance. Even with DYI antennas, or with external antenna, or with original PIF antennas.
With better antennas, the router is working fine with great range, BUT:
I can't set it to work in 2x2 MIMO mode. On 20 MHz channel, I get only 65/65 Mbit rates, and on 40 MHz (20 MHz HT Above/Below) I get only 150/150 Mbit rates.
I tried almost everything, different antenna setting (a, b, rxA/txB and txA/rxB) is not solving anything, configuration of the MCS rates also didn't work.
Of course there is chain 0 AND chain 1 enabled, rate selection advanced, security default or WPA2 AES, as mentioned in another topics.
I truly believe that RB751 can't be 2x2 MIMO, because in every antenna mode, it still uses only ONE antenna for TX, and there is no setting to use BOTH antennas for TX (or both for RX).
For 2x2 MIMO, the wireless card has to use TWO antennas, both in rx/tx mode - but RB751 can't
In my RB433 + R52Hn with two omni antennas and both chains enabled, there is no problem with 130/130 on 20 MHz channel. Also with any TPLink etc., it also runs fine with 130 Mbit rates on 20 MHz channel or 270(300) Mbit on 40 MHz channel.