MacOS Mojave 10.14.1 Captive Wi-Fi Problem

See Toggle Ability to turn wifi off which shows how to do so from the command line using a script:

#!/bin/sh

# Get "Wi-Fi" or "Airport" based on your OS
wservice=`/usr/sbin/networksetup -listallnetworkservices | grep -Ei '(Wi-Fi|AirPort)'`

# Get port (usually en1)
whwport=`/usr/sbin/networksetup -listallhardwareports | awk "/$wservice/,/Ethernet Address/" | awk 'NR==2' | cut -d " " -f 2`

# Set preferences
/usr/libexec/airportd "$whwport" prefs DisconnectOnLogout=Yes JoinMode=Automatic JoinModeFallback=DoNothing RememberRecentNetworks=No RequireAdminIBSS=Yes RequireAdminNetworkChange=No RequireAdminPowerToggle=Yes

This question had been answered previously, see Enable Admin Authorization to Change wireless Networks, also on jamfnation. Seems to be a common question for school IT admins.

I found these answers being curious about what you found by using the key from the preferences.plist file (RequireAdminPowerToggle) and using Google search.

Usually when the WiFi connection is still active but no data is flowing indicates Chanel crowding (to many devices/users on same channel).

If the WiFi is disconnecting that could be number of other reasons. But since you are talking about the speed going down...

When the MCS index is zero (0) then the channel crowding is a issue. The MCS needs to be at least 3 or more.

To see yours hold the "alt" key and click on the WiFi icon. (this is what I have)

Easiest test (fix) is to change the Chanel on the router.

You could also do a WiFi channel analysis to find out and then choose a less crowded Channel.

To change the Wifi channel on Pirelli DRG A125G router: