MBP 13" 2015 + high sierra with external monitor : wifi being spotty and dropping service irregularly I have a MPB 2015 13" MBP with High Sierra.
I bought a ViewSonic VP3881 monitor.
Monitor works flawless. (Except for a documented feature of sound control access from keyboard not working.)

But for some reason the wifi is being VERY spotty. I've had the monitor for 2 weeks now and I can't find an obvious pattern, except that it looses connection with the wifi on an irregular basis, while at the same time I have an older MacMini 2014 (El Capitain) on the same wifi, that does NOT drop the connection. Sometimes when rebooting it picks up the wifi, sometimes not even after reboot. Unplugging the monitor seem to make the wifi came back every time though.

Why would an external monitor interject with wifi?

This seem like a hardware problem, so I'm more or less forced to accept the situation, but hoping that there might be a checkbox somewhere I need to set to fix it?
External monitors can emit signals that interfere with WiFi signals. This is commonly due to insufficient shielding within the monitor. I would complain to ViewSonic - they might know about a design deficiency for this particular monitor, and have a solution or rebate at hand.

In order to try and fix this yourself, the most common way to solve this is simply to move your WiFi to a different set of frequencies, and hope that the monitor is not producing interference with that set of frequencies. It is usually hard for home users to shield the monitor properly without it being an ugly hack.

In essence you would need to access the settings on your WiFi access point and move it to a different channel. For example if you're using channel 11 now, try using channel 6 instead. You might need a few tries to find a sweet spot.

You can also switch from 2.4 Ghz WiFi to 5 Ghz WiFi if you have that available.