WIFI extremely slow after upgrading to "BCM94360HMB"

[voot566]

I have two iMac (iMac 20" 2008 early and iMac 27" 2009 Late")
Recently I bought two BCM94360HMB to upgrade the wifi for both machines.
However, the Wifi transfer speed within my HOME Network being extremely slow after upgrading. To transfer a 6GB file, it need more than an hour to do it, but if I do it with ethernet. It take less than 5 mins.
Additionally, the AIRDROP appeared in the system but it cannot find anything or being found.

All Mac machines are using SSD.

This is my trouble shooting steps:
A) I conducted clean install to both iMac. Everything being erased
B) 2008 iMac is running latest 10.11 El Captain with all update being patched. 2009 iMac is running the latest 10.13 High Sierra also with all updates from apple.
C) I have an Mac Mini 2014 inside the same room, which I tested to transfer the same 6GB file. The speed is normal. I didnt alter anything inside the Mac Mini except upgrading the SSD.
D) SPEEDTEST shown that the internet speeds are "UPLOAD 5x Mbps, DOWNLOAD around 2x Mbps on both iMac. I do the same SPEEDTEST on my Windows 10 PC, which it is 390 Mbps and 112.42 Mbps.
E) I also tried to test the 5G speed on my iPhone 12 Pro Max, and then share the internet to both iMac. It shows that the speed on my phone is 8x faster than my iMac on the same network in the same room.
F) The MacOS Wifi icon shows that my signal is in full bars

My router is QNAP Qhora-301W

Any ideas????
Such slow WIFI transfer speeds almost making my media share in my home network unusable.
The ISP provides 500mbps service plan to me. I am living in Hong Kong


This is the WIFI card I got.

BlackMagic Test between my iMac and my NAS:

SPEETEST result on my Windows PC:

SPEEDTEST result on one of my iMac

This is how I place my Antenna inside:

 

[nambuccaheadsau]

Fit three antennas and does your router use dual channels?

 

[voot566]

Fit three antennas and does your router use dual channels?

Yes, My router do support dual channel....... I pretty sure that all three antennas are properly installed. The left and right antenna are attached to the antenna come with the iMac, and the middle one is brand new which I attached to the fan holes on the top.
Additionally, I just did a TEST which, I disabled the smart connect on my router but break it down into two channels. My iMac can find and connect to the 2.4Ghz, with extremely slow transfer speed. But it is not able to find the 5G signal from my router, but the 5G signal from my neighbors?????

 

[voot566]

Fit three antennas and does your router use dual channels?

SOULTION:
I eventually disabled the SMART-CONNECT from my router. After separating the 2.4Ghz and 5.0Ghz Channels, I found out that my iMacs are unable to find the 5G signal from my router. It is because I have enabled MU-MIMO. That make my iMac always connecting to my router though the 2.4G Channel and that is why it is so slow
To solve this issue, just simply changing the the Bandwidth setting from my WIFI 5G to 80Mhz / 40Mhz / 20Mhz. Then the iMac found the 5G signal again. I re-enabled SMART-Connect, so now everything get back to normal........ my network speed back to 300mbps

 

[nambuccaheadsau]

See just had to get you thinking on the right lines.

Well done.

 

[voot566]

See just had to get you thinking on the right lines.

Well done.

Thank you so much~~~~