You will need to purchase the applicable amount ($30) in USD Xbox gift cards, these can be purchased from various online key stores e.g PCGamesupply, the current going rate being £11.47 for $15 USD cards. Depending on store used for purchasing credit and payment method used, small payment surcharges may apply.
Go to the FIFA 20 page on the US store and click the blue ‘Buy’ link and log in using your usual U.K. Gold account, this will first ask you to set up an account address for the US if you don’t already have one set up. Google is your friend. You are best using a non- tax paying state eg Oregon.
It will then ask you to add a payment option, you need to select the redeem code option at this stage. Don’t bother trying Revolut, Starling or Monzo etc - those payment methods won’t work. The purchase has to be completed entirely using US currency gift cards.
This doesn’t need to be completed via a VPN and you don’t need to touch your console settings either.
The game will then be available to download via your console as you normally would.