Not receiving newest base game items.

Th3Idi0tt  Try temporarily disabling your antivirus.  As long as you don't do anything else at the same time, your computer should be safe.  It's also worth deleting the content folder inside Documents > Electronic Arts > The Sims 4, where SDX content is located.

If disabling the antivirus does allow the SDX download, please let me know which AV you use, so I can add a note (for myself) about it interfering with the downloads.

If this doesn't help, can you test on a different network?  For example, you could set your phone as a mobile hotspot, using data rather than your home wifi, and connect to that.