Old thread, but issue is still the same...
I've had Asus RT-AC66 B1 with Merlin firmware (now 384.11_2) for a while now, and this is bugging the hell out of me.
I have 3 Samsung phones in the house, S5 , S7 Exynos and now S10 Exynos, and every one of these can only connect at 192 Mbit/s speed when using 2,4 Ghz network.
Changing router settings won't make any difference (channel, beamforming, airtime fairness etc.), always maxes to that, regardless of distance to router or settings being used. Ther's not other networks in the area, so air is "clean" of other traffic. Changing firmwares also won't make any difference.
Only thing that I can tell is somewhat strange at routers end is that even when 2,4GHz is used router reports ac PHY being used instead of n? Why?