Since you live in a EU country, I'm pretty sure that your signature has no juridical significance whatsoever, as clearly you have had no chance of inspecting the device at that point. You probably are just required (by the law) to complain during "a reasonable time", which might be several weeks. The bottom line is this: No matter what you sign, you still have your legal rights. You probably have some town officials for consumer protection affairs who are paid to help people with cases like this.