http server switching network interface after it connected to a wifi

It is probably just a case of modifying the codebase you are using. Here is some code that runs ESP8266 as station and access point.

let ap_ssid="test";
let ap_authMode="wpa_wpa2";
let ap_pwd="testtest";

let st_ssid="ssid";
let st_pwd="pwd";

let port=8080;

let http = require('http');
let wifi = require("Wifi");

let startServer = function(port){
  let server = http.createServer(function (req, res) {
      res.writeHead(200, {'Content-Type': 'text/html'});
      res.end(`AP IP: ${wifi.getAPIP().ip}</br>STA IP: ${wifi.getIP().ip}`);
  });
  console.log("server started");
  server.listen(port);
};

console.log("disconnecting all..");
wifi.disconnect();

// start ap
wifi.startAP(ap_ssid,{
  "authMode" : ap_authMode,
  "password" : ap_pwd
},(err)=>{
  if (!err) {
    console.log("AP started");
  } else {
    console.log("could not start AP"); 
  }
  
  // conect as station
  wifi.connect(st_ssid, {password:st_pwd}, (err) => {
    if (!err){
      console.log("station connected");
      startServer(port);
    }else{
      console.log("could not connect as station");
    }
  });
});

The server/page can be connected to on AP IP: 192.168.4.1 (port 8080) and the router allocated IP of 192.168.1.34 in this example.

Both the AP IP and station IP are displayed in the page.

AP IP: 192.168.4.1
STA IP: 192.168.1.34

Perhaps you can modify from here to suit your needs.