Debugging MQTT

Hi, am struggling to get MQTT working on ESP8266.
I have numerous actual Espruinos (eg Espruino Wifi), and this code runs fine:

const mqtt = require("tinyMQTT").create('.... url of local hivemq server ...');
mqtt.on('error', function(err){console.log(err)})
mqtt.on('connected', function(){console.log('connected')})

var wifi = require("Wifi");
wifi.connect("...", {password:"...."}, function(){
  console.log("wifi connected");
  mqtt.connect();
});
wifi.stopAP();

Am able to connect to the MQTT server fine with a real Espruino, but with ESP8266 I get nothing, not even an error.

This happens with the official MQTT library and tinyMQTT - work fine on Espruino WiFi, fails on ESP8266.

Question: how to debug this and figure out what's wrong?

David