Where is my game?

Thanks for the response but the ready to install section isn't showing. I've rebooted, I've unplugged it, I've signed out and in and it still isn't there and to be honest, I don't remember ever seeing it. I've tried a few threads on the internet to try and
fix it but nothing seems to be coming up.

I honestly don't know, I guess it depends on the size of the DLC and the developer and/or publisher and how much they wanna charge use for it.

Through the store it says product is installed with no other options.

Yeah I agree. Although the game you mention also shows up in Steam. May I suggest the o.p. check other game stores such as stream, as you may have purchased it there?

You can't email Microsoft directly, as they block "unsolicited" mails. Otherwise they would get flooded with spam mails, which would "drown" the legitimate mails.

In your case, you should go to
https://support.xbox.com/contact-us and try to contact the official support via their contact system.

Sign in with the Microsoft Account that you used to purchase the game, and click on the "Contact us" button. Then choose "Billing & Subscriptions" as the first option, and then "I don't recognize a billing..." as the second option.

That should give you the option to either chat with a support agent via text chat, or you can request a callback, where a Microsoft Agent will call you back on your phone.