If you have purchased Windows 10 End of Support (ESU) then you need to update the Keys on the respective devices in order to get windows updates. Keys are provided as MAK (Multiple Activation Key) and need to be distributed to the clients.
The ESU keys need to be purchased on an agreement. Its not avaliable self service or for consumers.

Keys are claimed from the licensing page in M365 Admin center and will show up in this list. NOTE: I don’t have ESU myself in this list. https://admin.microsoft.com/adminportal/home#/subscriptions/vlnew

If you don’t see the ESU key or no keys for that matter, or mabye even you don’t have access. Then you need to be added. That can be done by an administrator for the agreement or by your partner. Check access to view licenses? Click Invoicing->Your Products->Contracts then look for your agreement. Need to claim your volume license (VL) keys for Microsoft products?

Here is the full guide on how to obtain and deploy the MAK keys https://learn.microsoft.com/en-us/windows/whats-new/enable-extended-security-updates#get-the-product-keys-for-activating-extended-security-update-esu-licenses

No internet? No problem, use phone to activate the product. First apply the MAK key then ring in to Microsoft in activate. See link below:
https://learn.microsoft.com/en-us/windows/whats-new/enable-extended-security-updates#activate-esu-keys-by-phone

KMS activation is not avaliable for ESU keys.

Windows 10 ESU will double in price for the next two years.

Disclosure: Written at work for Microsoft Norway as a response to a customer question.