I spoke to a few other MBPro owners, and they said the had the same problem. I have not been able to reproduce this effect. Other people have said this is due to a design fault in the MBPro, and that if you push the USB-C connector all the way in, it shorts with the unibody and stops the wifi working. The wifi always instantly connects, just doesn't work. This seems to be nonsense, as moving the dongle close or far away has no effect - it can work or not work when the dongle is close, and work and not work when the dongle is far.

Some people have said that it is interference produce by the dongle. I called up the manufacturer of my favourite dongle (Wavlink), and they sent me a firmware update, but this had no effect. So what I do is I have to unplug, replug test, unplug, replug test, wifi off, wifi on, about 30 times until, after a random replug, it start working again. With the dongle plugged in, if I stop and start wifi, it instantly connects with 100% signal, but no internet. Plug the dongle in, wifi stops working, but stays with 100% signal. So I have to use a USB C to USB 3 dongle. All my devices are USB: mouse, keyboard, headphones, HD, USB stick, scanner, printer, SD card reader.

Bluetooth usb 3 port for mac