@Oi3B, let me provide two main steps (in general) that are needed (to get any supported non-free WiFi work under Debian 11 or equally under PureOS 10 with few adjustments), only those two steps needed for ThinkPad T420 (I’m quite sure, almost 100%):
Edit (open) your /etc/apt/sources.list like this:
Add to it below line, save and close sources.list file:
Please follow/reuse those two posts of mine (open them into new Tabs):
https://forums.puri.sm/t/wifi-on-macbook/14626/17
https://forums.puri.sm/t/wifi-on-macbook/14626/51
Not to forget (like myself) what else important:
In accordance to: Lenovo Thinkpad T420 - Gentoo wiki it is expected, after eventually successful completion of the above two steps (posts within this Forum), that for Centrino Advanced-N 6205 AGN following package will be installed/added to your PureOS installation:
apt list firmware-iwlwifi Listing... Done firmware-iwlwifi/stable,now 20210315-3 all [installed]
After above done please execute: sudo reboot and let us know if … if any further help needed.
P.S. If you like please write down your steps and post here those that might help to other people interested on how sudo isenkram-autoinstall-firmware command worked for you.