I'm going to start by asking the extremely stoopid question...
In your CM Console, All Software Updates, if you search for (for example) 1809, you DO see something like "2024-11 Cumulative Update for Windows 10 Version 1809 for x64-based Systems (KB5046615)"
We have a few 1809 LTSC, and that is the specific update that those Windows 10 1809 LTSC boxes want in my environment. Asking because perhaps what you are missing in your CM environment is enabling the Product of "Windows 10 LTSB" within your Administration, Site Configuration, Sites, right-click / Configure Site Components, Software Update Point, the Products tab.