Add support for newer mobile OS changes.

Took me quite a while to figure this out, but according to this issue (espressif/arduino-esp32#1037), in order to get a captive notification to show or the popup to open, the DNS server must resolve to a public IP. It will not work with a pivate one (e.g. 192.168.4.1). On Android, a notification ("Register with Network") is displayed in top left notification bar. On IOS, the login popup is displayed.
esp8266 · d-a-v · Jul 8, 2019 · Dec 19, 2018 · Dec 19, 2018 · Feb 14, 2019
commit d5265aa5ac4995b65bce3e29025307cfd7ae0d0f
diff --git a/libraries/DNSServer/examples/CaptivePortal/CaptivePortal.ino b/libraries/DNSServer/examples/CaptivePortal/CaptivePortal.ino
@@ -3,7 +3,7 @@
 #include <ESP8266WebServer.h>
 
 const byte DNS_PORT = 53;
-IPAddress apIP(192, 168, 1, 1);
+IPAddress apIP(172, 217, 28, 1);
 DNSServer dnsServer;
 ESP8266WebServer webServer(80);