For some reason I can not get a homebrew armor to accept the full DEX MOD.  I selected Full Dex in the dropdown and have even tried setting the Max Dex Armored AC and more in the modifiers.  it either still only adds 2 or it adds the Dex and the +2.  Help

It may not work now, or may work differently than before: but last time I needed to do it I had to use both the dropdown and the Modifier together.

I tried that, for some reason it still kept the 2 max

Please see the General Homebrew FAQ, question #6: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq#GeneralHomebrewFAQ&BasicTroubleshooting).