Google Analytics and Google Optimize expert needed

We are in need of some help from a Google Analytics and Google Optimize expert to help us track conversions and transaction across a root and subdomain. For example: "domain.com" and "checkout.domain.com"

Majority of the setup is complete using Google Tag Manager and we can see the A/B experiments being tracked successfully in Google Analytics but we aren't seeing the conversions/transactions.

We have setup Goals within Google Analytics to track when the user lands on the 'Order Complete' page but this doesn't seem to track it as a conversion.

We'd like to jump on a call with someone so we can explain the current setup in more detail as this will be easier than communicating over email or messages.