Deploy Flask app: static and templates not found

I am trying to deploy a Flask web to a Debian server. The files' structure is the following:

run.py
config.py
requirements.txt
/app
    constants.py
    errors.py
    forms.py
    __init__.py
    models.py
    routes.py
    /static
        /css
            /vendor
                bulma.css
        /js
            /vendor
                fontawesome.js

    /templates
        index.html
        login.html
        404.html

In order to deploy it on the server, I create a virtualenv and install the requirements.txt. Then, I start the web running the Python file run.py. When then I try to load the index page or the login page, it is unable to find any of the routing paths (although it works fine locally). Also, it is unable to locate the CSS and JS files. Instead of that, it fires the error handler on errors.py, raising an HTML error 404. The content of the files where I think the problem might be located is listed bellow:

Content of run.py

from app import app, db, socketio
from flask_assets import Environment, Bundle
from flask import Flask

if __name__ == '__main__':
    assets = Environment(app)
    assets.register('mainCss', \
        'css/vendor/bulma.css', filters='cssmin')
    assets.register('mainJs', \
        'js/own.js', 'js/vendor/fontawesome.js', filters='jsmin')

    socketio.run(app, debug=False, port=80, host= '0.0.0.0')

Content of config.py

import os
basedir = os.path.abspath(os.path.dirname(__file__))


class Config(object):
    SECRET_KEY = os.environ.get('SECRET_KEY') or 'you-will-never-guess'
    SQLALCHEMY_DATABASE_URI = os.environ.get('DATABASE_URL') or \
        'sqlite:///' + os.path.join(basedir, 'app.db')
    SQLALCHEMY_TRACK_MODIFICATIONS = False
    MAIL_SERVER = os.environ.get('MAIL_SERVER')
    MAIL_PORT = int(os.environ.get('MAIL_PORT') or 25)
    MAIL_USE_TLS = os.environ.get('MAIL_USE_TLS') is not None
    MAIL_USERNAME = os.environ.get('MAIL_USERNAME')
    MAIL_PASSWORD = os.environ.get('MAIL_PASSWORD')
    ADMINS = ['your-email@example.com']

Content of /app/__init__.py

import logging
from logging.handlers import SMTPHandler, RotatingFileHandler
import os
from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_migrate import Migrate
from flask_login import LoginManager
from config import Config
from flask_socketio import SocketIO
from threading import Lock

app = Flask(__name__)
app.config.from_object(Config)
db = SQLAlchemy(app)
migrate = Migrate(app, db)
login = LoginManager(app)

thread = None
thread_lock = Lock()
socketio = SocketIO(app)

from app import routes, models, errors

Content of /app/routes.py

from flask import render_template, flash, redirect, url_for, request
from app import app, db
from app.forms import LoginForm

@app.route('/login', methods=['GET', 'POST'])
def login():

    form = LoginForm()
    if form.validate_on_submit():
        #Validate login
        return redirect(url_for('index'))
    return render_template('login.html', title='Login', form=form)

@app.route('/index', methods=['GET', 'POST'])
def index():
    return render_template('index.html', title='Index')

Therefore, my question is: why does this work locally but it fails to find the paths of the static and template files when deployed to a Debian server and how can I solve this issue?