Hi @nyazilla
I have some suggestion,
try updating the drivers and see if it works.
https://downloadcenter.intel.com/product/59474/Intel-Centrino-Ultimate-N-6300-Dual-Band
Install Hotkey Features Integration under Software and Utilities
https://pcsupport.lenovo.com/sg/en/products/laptops-and-netbooks/thinkpad-t-series-laptops/thinkpad-t420/downloads?linkTrack=Caps%3ABody_SearchProduct&searchType=3&keyWordSearch=T420%20Laptop%20(ThinkPad)
If it does not work, try blocking pin 20 and see if it works.
Sometimes faulty wifi switch can cause this weird issues.
https://www.eevblog.com/forum/beginners/laptop-mini-pci-_off_-switch-to-defeat-bios-restriction-on-choice-of-wifi-cards/