Microsoft were providing updates for Secure Boot Certificated in May and June, then they realized there was a problem with the updates caused by the OEM PC manufacturers so they have paused the secure boot updates and they are working with the OEM manufacturers to resolve the problem, they will then begin the Secure Boot updates again.
If your PC hardware is too old and Dell do not provide a new BIOS update, then you may not receive the secure boot updates, Windows will continue to work normally and there is no other process available, even a 3rd party anti-virus will not protect the firmware-level trust database handled by Secure Boot.