-Acağı yerde    [ instead of ]

Verb stem + acağı + personal ending +   yerde [eceği yerde ]

Gerund of  replacing actions.

* This form tells us the unwanted second action is replaced with       the desired first action.

*  Two actions that are oppossing to eachother , expressing a displeasure for the wrong preference [choise].

* This form is used with only actions.

- Bana teşekkür edeceği yerde hakaret ediyor.

   Instead of thanking me , she is insulting me.

   to thank  -  to insult    have  opposite concepts.

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 Onunla konuşacağın yerde  kavga  ediyorsun.

 Instead of talking to him, you are fighting with him.

  to talk  -  to fight

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Evde oturup ders çalışacağın yerde geziyorsun.

Instead of staying  home and studying, you are hanging out.

to stay home and  study -  to  hang out

Wouldn´t simple DAT work here as well?

                        ?Onunla konuşacağına kavga ediyorsun.

Yes, simple dative  works  as well !  and it gives us the same meaning