先ず Debian 環境に Flask をインストール．
pip3 install Flask

サンプルの hello.py を実行，ブラウザで表示．

AS5202T でも同様に Flask をインストール．
pip3 install Flask

Synology NAS では pip のインストールから．
suto opkg install python-pip
suto opkg install python3-pip

Flask をインストールしてみたが …

Iwao@DS116:~/pyt_test/t_flask$ pip3 install Flask
Collecting Flask
  Using cached https://files.pythonhosted.org/packages/f2/28/2a03252dfb9ebf377f40fba6a7841b47083260bf8bd8e737b0c6952df83f/Flask-1.1.2-py2.py3-none-any.whl
Collecting click>=5.1 (from Flask)
  Using cached https://files.pythonhosted.org/packages/d2/3d/fa76db83bf75c4f8d338c2fd15c8d33fdd7ad23a9b5e57eb6c5de26b430e/click-7.1.2-py2.py3-none-any.whl
Collecting itsdangerous>=0.24 (from Flask)
  Using cached https://files.pythonhosted.org/packages/76/ae/44b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1.0-py2.py3-none-any.whl
Collecting Werkzeug>=0.15 (from Flask)
  Using cached https://files.pythonhosted.org/packages/cc/94/5f7079a0e00bd6863ef8f1da638721e9da21e5bacee597595b318f71d62e/Werkzeug-1.0.1-py2.py3-none-any.whl
Collecting Jinja2>=2.10.1 (from Flask)
  Using cached https://files.pythonhosted.org/packages/30/9e/f663a2aa66a09d838042ae1a2c5659828bb9b41ea3a6efa20a20fd92b121/Jinja2-2.11.2-py2.py3-none-any.whl
Collecting MarkupSafe>=0.23 (from Jinja2>=2.10.1->Flask)
  Using cached https://files.pythonhosted.org/packages/b9/2e/64db92e53b86efccfaea71321f597fa2e1b2bd3853d8ce658568f7a13094/MarkupSafe-1.1.1.tar.gz
    Complete output from command python setup.py egg_info:
    /opt/lib/python3.6/distutils/dist.py:261: UserWarning: Unknown distribution option: 'project_urls'
    /opt/lib/python3.6/distutils/dist.py:261: UserWarning: Unknown distribution option: 'include_package_data'
    /opt/lib/python3.6/distutils/dist.py:261: UserWarning: Unknown distribution option: 'python_requires'
    usage: -c [global_opts] cmd1 [cmd1_opts] [cmd2 [cmd2_opts] ...]
       or: -c --help [cmd1 cmd2 ...]
       or: -c --help-commands
       or: -c cmd --help

    error: invalid command 'egg_info'

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-dsqn65uh/MarkupSafe/
Iwao@DS116:~/pyt_test/t_flask$ 

「 Command python setup.py egg_info faild with error code 1 」で検索 をかけると pip などが古いとある．
次の様にして pip などを更新．
sudo pip install –upgrade pip setuptools