Service après vente nul

I bought 4 Fossibot F2400 Power Stations and one Oukitel P501 from them (so orders for more than 5000 Euros in total).
One of the Fossibot does not work properly in UPS mode and sometimes stops when it is charged or when EDF cuts and, therefore, it should switch to the battery in automatic. I reported it after 2 weeks and I could not get it exchanged.
They ask me for videos (I wonder how to film since I don’t know in advance when it will happen) and we look for excuses not to exchange it and drag it out without resolution of the problem.
This is a site to avoid if you expect quality after-sales service in case of problems with one of their products.
Since I have accumulated negative comments on several sites, they offered me a voucher to return the defective Station more than a week ago, but nothing is arrived since. Probably a lie to make me wait.
I sincerely regret having ordered on a Chinese site one day, they do not only know how to sell at low prices without any after sales service.
To be avoided!