what to do if moudle not in pip3 only in pip2?

[snippsat]

Something is messed up.
Requirement already satisfied: modem-cmd in ./.local/lib/python3.7/site-packages (1.0.2)
Requirement already satisfied: pyserial>=2.6 in /usr/lib/python3/dist-packages (from modem-cmd) (3.4)
See that it also mention /usr/lib/python3

And you shall not do pip3 install serial that and other old package(can make trouble) that's not pyserial 3.5
The only install needed is pip3 install modem-cmd(will install also pyserial 3.5)

Don't what to help with now something is messed up other that suggested virtual environment.

# Make 
python3.7 -m venv modem_env

# Cd in
cd modem_env/

# Activate see that it say (modem_env) first now
source bin/activate
 
# Install
pip3 install modem-cmd
Collecting modem-cmd
  Downloading modem-cmd-1.0.2.tar.gz (2.2 kB)
Collecting pyserial>=2.6
  Downloading pyserial-3.5-py2.py3-none-any.whl (90 kB)
     |████████████████████████████████| 90 kB 1.6 MB/s 
Using legacy 'setup.py install' for modem-cmd, since package 'wheel' is not installed.
Installing collected packages: pyserial, modem-cmd
    Running setup.py install for modem-cmd ... done
Successfully installed modem-cmd-1.0.2 pyserial-3.5

Test that i works python3.7 for you

$ python
Python 3.9.6 (default, Aug  3 2021, 16:49:17) 
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from modemcmd import modemcmd
>>> import serial
>>> 
>>> modemcmd
<function modemcmd at 0x7f774ea65ee0>
>>> 
>>> serial.__version__
'3.5'
>>> exit()

Can also look at pyenv Simple Python Version Management.
That what i use then it easy to install versions,as Python3.7 start to get old now.