Nor did I... but in any case it's true... I would imagine when you change the security settings the router restarts it's wifi and probably DHCP server, which sometimes requires the devices and/or the app to be restarted. Because TP Link discovery is non-standard be sure to force your phone onto your 2.4GHz SSID when searching. If they're flashing orange (or whatever they flash when they're not able to connect to wifi depending on type) I'd try restarting them, and if that doesn't work then restart the router, and perhaps the devices once again. If they still won't reconnect to wifi it might be a question for your router manufacturer.