Use another device that has an internet connection to download this file:
If you installed PureOS 10.0 Byzantium:
http://ftp.us.debian.org/debian/pool/non-free/f/firmware-nonfree/firmware-iwlwifi_20210315-3_all.deb
If you installed PureOS 9.0 Amber:
http://ftp.us.debian.org/debian/pool/non-free/f/firmware-nonfree/firmware-iwlwifi_20190114-2_all.deb
Save that file on some kind of portable storage device (USB Flash memory drive, external harddrive/SSD, mobile phone, etc.).
Then connect your portable storage device via USB cable to your Thinkpad. If your storage device supports USB On-The-Go, then it should auto-mount so you can see its files on your Thinkpad and you can figure out the path to get to it. If you don’t see anything, open Nautilus, press F9 to see the sidebar and look under “Devices”. Usually USB drives auto-mount at /media/{username}/{device-name} and Android devices mount at /run/user/{user-id}/gvfs/mtp:host={device-name}. If using an Android phone, remember to go to “USB Preferences” on the phone and select the option to transfer files after plugging your phone into your Thinkpad.
Then open a terminal on your Thinkpad and use the cd command to change to the directory where the firmware-iwlwifi_20210315-3_all.deb file is stored. Then issue this command to install it:
sudo dpkg -i firmware-iwlwifi_20210315-3_all.deb
After that, check if the WiFi is functioning. You may have to reboot. You can use sudo iwlist scanning to check if it is working.