The compatibility of the "ThinkPad Fibocom L860-GL-16 4G LTE CAT16 M.2 WWAN Module" with your Lenovo X1 Carbon 11th Gen (Type 21HM) is guaranteed, as it was specially developed for the X1 Carbon Gen 11. If the module is not compatible for any reason, you can usually return it, provided that it is in an unused condition and the retailer's return policy conditions are met. It is advisable to check the retailer's returns policy to ensure you get your money back.