Cross domain tracking with GTM and React

I am using GTM to instrument Amplitude and have an issue I think many of you have bumped into. I am trying to track a user across different domains by passing the deviceid in the url.

However, I can't access the deviceid in the right amplitude instance from code in order to pass it in the url. Here's a reference to a description of the problem in an Amplitude Community forum: https://community.amplitude.com/data-instrumentation-57/cross-domain-tracking-typescript-1461?postid=8608#post8608

I'd be grateful for any help as this is going to be crucial for our implementation to work. Thanks! (ping @Esther Trapadoux)