Que and qui are pronouns that replace someone or something (like “that” in this very sentence):

• Que is for something, and also for someone when it replaces the object of the relative clause.
• Qui is for someone when this someone is the subject of the relative clause.

In your case, 2 possible approaches:

1. You ask yourself what the blank word replaces in the sentence. What did you buy? A device. Something. Hence: que, with no doubt.
2. You ask yourself what’s the grammatical type of the object represented by the pronoun: here, it’s an “object” (You bought XXX). So, again, no question: even if the object would have been a person, the answer is que.

As the pronoun replaces a thing + the grammatical type is “object”, there are here two reasons for the answer to be que.

As a side note: about “que tu as acheté” in your sentence. I’m not 100% sure this is totally incorrect, but I’m pretty sure a better mode is “que tu aies acheté” (i.e. use the subjunctive). You may check a long debate on this here.