So after hitting my head in the wall on this issue also on and off for quite some time and i finally by accident figured out that even though my pos printer support https it does not in fact support it for printing, only the web interface for management for some strange reason... so i made a small proxy script to downgrade https to http...

make a directory of your choice then...

Commands:

-npm install express http-proxy-middleware https

-openssl req -nodes -new -x509 -keyout server.key -out server.cert

-nano proxyServer.js (and copy/paste and edit from below)

const fs = require('fs');

const https = require('https');

const express = require('express');

const { createProxyMiddleware, responseInterceptor } = require('http-proxy-middleware');

const app = express();

// Configuration variable for the target host

const TARGET_HOST = 'http:xxx'; // Replace with your target host

// Proxy middleware options with response handling

const options = {

  target: TARGET_HOST,

  changeOrigin: true,

  selfHandleResponse: true, // The proxy will manage the response

  onProxyRes: responseInterceptor(async (responseBuffer, proxyRes, req, res) => {

    // Modify or translate the response from the printer here

    // For example, you can change headers, status code, or the response body

    const response = responseBuffer.toString('utf8'); // Assuming the response is a string

    // Perform necessary modifications to the response

    return response;

  })

};

// Use the proxy middleware

app.use('/', createProxyMiddleware(options));

// HTTPS server with self-signed certificate

https.createServer({

  key: fs.readFileSync('server.key'),

  cert: fs.readFileSync('server.cert')

}, app).listen(443, () => {

  console.log('Listening on port 443 (HTTPS)');

});

then run: 

node proxyServer.js

update for reference:

made a python minimalistic version also
https://github.com/kimasplund/minimal-proxy

works on my TM-T88V with UB-R04 wifi interface board...

Open the developer mode in the pos screen and click on the network tab. Then click on the printer icon or make a payment and check the responses of the network calls.This will lead to some clues