Appleoddity's answer gives everything you need. Some caveats though;
- Update-MpSignature never worked for me in powershell. I spent a lot of time trying to set up the environment for these updates to work and was using Update-MpSignature as the test.
- Once I actually run Defender/MSE's built in update function and realized that the update worked correctly in a situation where Update-MpSignature was failing I started backtracking and testing other scenarios which I though weren't working because they were failing through powershell
So basically, as you work through the issue using Appleoddity's guide make sure not to rely solely on powershell and Update-MpSignature to test what you're doing. YMMV but it my case I was never able to succesfully run Update-MpSignature. I had jumped to the conclusion that I had set something up wrong, but after more testing I saw that defender itself was updating without issue and only powershell was having problems.
