Q: Mac Pro (Late 2013) WiFi Problem

make sure IPv6 is set to "link-local only" or it will interfere with normal (IPv4) operation.

hold down the Option key while you click on the Wi-Fi Icon on the menuBar. You get a daily similar to this older one:

what do you get for PHY mode and channel?

what do you get for RSSI, signal strength, or signal to noise?

what do you get for Transmit rate?

and very important, how many other networks do you see?

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You are using the very crowded and interference-prone 2.4GHz band. With more that three other networks visible, you will begin to have problems here because the channels are very narrow, and it takes five of them (centered about the nominal channel) to send your data using 802.11g or 802.11n.

for signal, you have -58 -89 yields -31dB, a truly excellent signal-to-noise, and your internal antenna is not disconnected. Transmit rates is as fast as you can go in this band.

But with 13 other networks visible, you should move to the 5GHz band to get away from your neighbors. If your Router does not offer 5GHz band, you may need to get an additional Router or a replacement router. In some cases, the 2.4GHz band and the 5GHz band can be named as different network-names, to allow you to connect ONLY to the 5GHz band for better rates.

additional diagnostics are available by choosing "Wireless Diagnostics" off the Option-Wi-Fi menu -- not to be confused with the much simpler diagnostic available with:

system preferences > Network > Wi-Fi > (assist me) > (diagnostic)

Once the Wireless diagnostic has run through once and given you its broad recommendations, you can continue to leave it running in the background, and if unusual events occur, it will notify you. There are also additional troubleshooting tools available inside it, as described in this article, but: DON'T Save a report. It is a debugging snapshot of every possible item that could influence Wi-Fi performance in any way. It is HUGE, impenetrable, and useful only to a diagnostic engineer.

Use Wireless Diagnostics to help you resolve Wi-Fi issues on your Mac - Apple Support

(most article are available in many languages)

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The settings you are showing are only for the 2.4GHz band, unless you can set the channel on that page higher than 20 (typical 5GHz channels run (country-dependent) from about 35 to about 165).

In the Apple Airport, you can enable and name the 5HGz band separately, which allows you to connect to it separately. Both bands continue to operate simultaneously.

In the UK, one of the default Routers provide by British Telecom can switch between 2.4GHz and 5GHz band, but cannot operate on both bands at once.

A frequent surprise that US Users sometimes encounter is that they get better service from a nearby, unencrypted Router their neighbor has left open.

What options are available under Mode?

As Grand said it is likely that with mixed mode you only get 2.4 GHz since since b and g are only 2.4 GHz while n is both 2.4 and 5 GHz

802.11n is backward-compatible with many devices.

Older printers and really old computers MAY be left behind, but try it and see -- it is often Not a problem, and you will quickly know what is happening if the older devices drop out.

It looks like that Router in not capable of 5GHz. What brand and model is it? sometimes we can look up the capabilities online.

you can also set up a slightly more complex network, but these work best when you can connect the Routers together with ethernet cables:

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Based on the info yo provided and the manual

http://setuprouter.com/router/zte/h220n/manual-923.pdf

I conclude that your router only supports the 2.4 GHz frequencies

Some of those others are using different Wi-Fi chips.

this graphic was posted by a user. It shows just how W-I-D-E the channels in the 2.4GHz band are, and what a mess ensues when you ty to cram too many into the 2.4GHz band:

(click to enlarge)

the graphic is from an inexpensive utility called Wi-Fi Explorer, available from the Mac App Store for not much money, with a three-day free trial.