If you find yourself in a situation like this in the EU, where consumer protection is strong, you should talk to the consumer rights office in the country where you bought the item, or where you were when signing the sale contract.
First, they will tell you how to complain to the company - what specific terms to use, and how long they have to respond and to resolve your matter.
Second, if you don't get a proper resolution, they will advise you how to sue the seller. You will usually have law on your side by that point, so there is almost no way the seller can weasel out.
Third, the consumer rights office you talk to might have contacts for expert investigators who can act as a third party in your court case. They will provide a written statement to the effect of "this is what happened, this is the law, the buyer can demand this remedy".
Finally, the seller will likely settle out of court, because you're not the first customer they abused and they already know how these things go. In the EU, you will often get either a replacement of the item, or a full refund, or sometimes you might get to choose. That is the bare minimum, you can also demand that they pay your court costs in most EU countries, and some compensation for loss of income, cancelled holidays, and other such things. Depending on how strong your negotiating position is, you will get some of these things.
If you have a website about how bad the product is and you are willing to take it down or write about reaching a satisfactory conclusion after negotiations with the seller, your negotiating position is very strong.
The consumer rights office will tell you all this process and it will vary a little bit. But it's important to do this relatively without delay, there is no point in dealing with unresponsive customer service. You need to tell them the right things by law and they will be forced to respond or to face the lawsuit. An abusive company will not respond out of good will, without compelling them to, you are just wasting time.
In Holland, the consumer rights office is the ACM.