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

[snippsat]

It now read from wrong package,it shall read from pyserial and then sub-folder serial.
Rename or delete python3.7/site-packages/serial/ folder and try again.

You could of course use virtual environment,it's build into Python and then will avoid conflict like this.
Example.

tom@tom:~$ python -m venv modem_env
tom@tom:~$ cd modem_env/
tom@tom:~/modem_env$ source bin/activate

(modem_env) tom@tom:~/modem_env$ pip 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

(modem_env) tom@tom:~/modem_env$ 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
>>> 
>>> modecmd
<function modemcmd at 0x7f6c98baaf70>