How to Use Rest api with React Native; Network Call Issues

I am newbie in React Native, I made a simple back-end using Mongodb and express routes etc in MongoDb atlas. I am successfully able to post/get/patch/Delete operation on mongodb atlas that store Title and Description using Postman. Everything is working fine.

Here comes the problem First when i make a simple frontend in ReactNative that take inputs Title and Description. I want application that take simple input of Title and Description and on Submit Button it store into the the mongodb Atlas just like postman is doing. I tried but its not working code is below. I dont know how to communicate the front end into backend. I watch alot of tutorials but unable to get the point.

Secondly, when i make a server i wrote in pakage.json > "start": "nodemone server.js" and i need to run ReactNative app i update the pakage.json > "start": "expo start" to run app. How can i run server and expo app same time? if i seprate the app folder then how can i connect both of them. below is my Code.

Routes folder post.js

const express = require( 'express' );
const router = express.Router();
const Post = require ('../models/Post')

//Gets back all the posts
router.get ( '/', async (req, res) =>{
    try{
      const post = await Post.find();
      res.json(post);
    }catch (err) {
      res.json({message: err })
    } 
  });

//To Submit the Post
router.post('/', async (req, res) =>{
  //console.log(req.body);
  const post = new Post({ 
    title: req.body.title,
    description: req.body.description
  });
  try{
    const savedPost = await post.save();
    res.json(savedPost);
  }catch (err) {
    res.json ({ message: err })
  }
});

//Get back specific Post
router.get('/:postId', async (req, res) =>{
  try{
  const post=  await Post.findById(req.params.postId);
  res.json(post);
  }catch(err) {
    res.json({message: err });
  }
})
// to delete specific post 
router.delete('/:postId', async (req, res) =>{
  try{
  const removePost=  await Post.remove({_id: req.params.postId});
  res.json(removePost);
  }catch(err) {
    res.json({message: err });
  }
})

//update Post
router.patch('/:postId', async (req, res) =>{
  try{
  const updatePost =  await Post.updateOne(
    {_id: req.params.postId}, 
    { $set: 
      {title: req.body.title}
    }); 
  res.json(updatePost);
  }catch(err) {
    res.json({message: err });
  }
})

module.exports = router;  

Defined Schema Post.js

const mongoos = require( 'mongoose' );

const PostSchema = mongoos.Schema ({
    title: {
        type: String,
        required: true
    },
    description: {
        type: String,
        required: true
    },
    date: {
        type: Date,
        default: Date.now 
    }
})

module.exports = mongoos.model ('Post', PostSchema); // giving this schma name Post  

server.js

const express = require( 'express' );
const app = express();
var mongo = require('mongodb');
const mongoos = require( 'mongoose' );
const bodyParser = require('body-parser');
require('dotenv/config');
const postRoute = require('./Routes/post');

app.use(bodyParser.json());
app.use ('/post', postRoute);

app.get ( '/', (req, res) =>{
  res.send('We are on Home ')
});


// connecting to database
mongoos.connect(
  process.env.DB_CONNECTION, 
  { useNewUrlParser: true },
  () => console.log('Connected to db')
);

app.listen(3000);

Frontend Form.js

import React from 'react';
import { StyleSheet, Text, View, TextInput, TouchableOpacity } from 'react-native';



class Form extends React.Component{
    constructor(){
        super();
        this.State = {
            title: '',
            description: ''
        } 
    }

    getInput(text, field){
        if(field == 'title')
        { 
            this.setState({ title: text, })
        }
        else if(field == 'description')
        {
            this.setState({ description: text, })
        }
        //console.warn(text)
    } 

    submit(){
        let collection={}
        collection.title = this.state.title,
        collection.description = this.state.description;
        console.warn(collection);  
        var url = process.env.DB_CONNECTION ;
        fetch(url, {
            method: 'POST',
            headers: {
                Accept: 'application/json',
                'Content-Type': 'application/json',
            },
            body: JSON.stringify({
                collection
            }),
        });
    }

    render() {
      return (
        <View style={styles.container}> 

            <TextInput style={styles.inputBox} 
              underlineColorAndroid= 'rgba(0,0,0,0)'
              placeholder='Title'
              selectionColor="#fff" 
              keyboardType="default"
              onChangeText = {(text) => this.getInput(text, 'title')}
            />

            <TextInput style={styles.inputBox} 
              multiline = {true}
              numberOfLines = {4}
              underlineColorAndroid= 'rgba(0,0,0,0)'
              placeholder='Description'
              selectionColor="#fff" 
              keyboardType="default"
              onChangeText= {(text) => this.getInput(text, 'description')}
            />

            <TouchableOpacity onPress={()=>this.submit()} style={styles.btn} >
                <Text style={{textAlign: 'center'}}>Submit</Text>
            </TouchableOpacity>

        </View>
    );
    }
}  

export default Form;