Your raw signal level (RSSI) is not very good to start with.
Values like -40 right next to your Router, and values like -75 are completely unusable. Your -71 is at the point where your Mac is evaluating whether other networks nearby could be used instead.
your noise level suggests there are other Routers using your same or nearby channels.
Number of Streaming Servers (NSS) of 1 says you are only using one of your two antennas, likely because the signal is not clear of interference from adjacent channel used by others nearby.
Ideally, your should:
• reduce the distance and intervening building materials between your Router and your computer. It may be easier to move the Router than the computer. Cable TV cable could be extended, or Ethernet cable between modem and Router could be made longer.
• inside your Router, be certain that channel selection is "automatic", if available. Then cycle the power to your Router to get it to sample what is already in use, and switch automatically to a less-used channel.
you can also use an inexpensive utility like WiFi explorer to look at what is actually around you there should be no overlaps if possible:
in this counter-example, everyone in the 2.4 GHz band is having a bad day, and the 5 GHz band is slow and disconnecting often. manually or automatically moving to a different 5 GHz channel would solve the problems there.
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if you are desperate, and can not do any of the other suggestions, it is possible that reducing the channel-width to 40 MHz might actually produce slightly better results using two antennas.
