Tags usability through custom field

Hi!

I am using a CPT which I want to be displayed in the tag archives. But, I don’t want the default tag picker which forces you to type tags; I want users to only pick tags from a list, so instead of the default tags support, I’m using a custom relationship field in my CPT linked to the tags taxonomy which displays a pretty list with checkmarks. The problem is that tags assigned this way are not recognized by the default tag archive. How do I make the tag archive look at this custom field for tags too?

Thanks in advance

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