Can't Associate Existing Contacts to an Account in the D365 UCI? - WHERE IS THE DAMN BUTTON??

Don't feel like the emotional post? :D :D FIND THE SOLUTION HERE!!

Recently I was doing some config. within Dynamics 365 for a customer that was keen on using the UCI (Unified Client Interface).  I've done a load of demo's for this type of thing but never actually helped configure for a live implementation. To be deadly honest, I had to change my mindset a bit about how I designed things.

After a few hours of build i ended up setting up a little training solution that allowed users the ability to associate existing contacts to an account. A pretty straight forward N:N relationship on the account form showing the associated contacts that had been trained and the internal users that had actually done the training.

I was happy with the setup but when it came down to testing, something weird happened... There was no button available on the sub grid that allowed me to associate an existing record. The Add Existing User button was there, but no "Add Existing Contact" button :( 

I checked if it was working in the Web refresh... Lo and behold, i could work just fine! No issues with adding existing contacts to an account. 

I tested out different views and still no luck I even tried a 1:N relationship to Contact and it only allowed me to create a NEW contact rather than add an existing one. 

After a bit of searching... I found the one solution that worked! THANK YOU to the Joel & THE TIP OF THE DAY team for this post! you saved me a TON of time. This could have been a rabbit hole. Also Scott, your Ribbon Workbench comes to the rescue again! Legends!

FIND THE SOLUTION HERE!!