I see your upload is very, very low.
I'm sure you have a lot of retransmission on uplink.
Retransmissions in uplink impacts your download because of TCP traffic nature that requires ACK.
You clearly have interferences on the AP on the frequency of your AP and not on SM side (because your download speed is high).
Please do a spectral scan to check this on both side.
If you can't do better because it's the "best" frequency possible, I suggest you to look at the "Performances" section on the SM in Uplink, and look at the MCS that has the highest percentage of traffic.
Set your SM Uplink to that MCS as a fixed value: your TCP traffic will go up and your link will become more stable.
I noticed that the MCS selection algorithm doesn't work so well in high interferences environment and it's better to manually set uplink to have a good result in TCP speed tests on speedtest.net.
I also see your signal is very high (-44), so maybe you have saturation due to high signal. If your spectrum is clean, try to lower the power or to enable auto power on the AP.
If you don't have frequency reuse enabled and you're using auto power, you can set your target signal to a higher value than the default -60dBm to have a higher difference between noise and signal.
You can also use eDetect to show co-channel interferences on both sides of the link.
I'm sure you'll solve your issue :-)