ModuleNotFoundError: No module named 'pip'

I mount /dev/sda1,install python3-pip in usb.

opkg update
opkg install python3-pip  -d usb

I want to install sphinx with pip:

root@GL-A1300:~# ls  /mnt/sda1/usr/bin
pip     pip3    pip3.9
root@GL-A1300:~# pip   install sphinx 
Traceback (most recent call last):
  File "/mnt/sda1/usr/bin/pip", line 5, in <module>
    from pip._internal.cli.main import main
ModuleNotFoundError: No module named 'pip'
root@GL-A1300:~# /mnt/sda1/usr/bin/pip   install sphinx 
Traceback (most recent call last):
  File "/mnt/sda1/usr/bin/pip", line 5, in <module>
    from pip._internal.cli.main import main
ModuleNotFoundError: No module named 'pip'

How can fix it?