Poor after service - no way of contacting him to exercise our right to the guarantee he offers

I have been trying to contact Michael, the owner of the company, since the beginning of December last year, but I have received no answer from him. I have tried phoning him, messaging him and sending him messages to Whatsapp with no luck. He did a cementitious waterproofing at our balcony at the end of August last year and promised that the leak problem we had would be solved after applying his product, but after the first rains the ceiling got watermarks again... He offers a 5 year guarantee on this product, but what is the point if we cannot get hold of him? We are not happy with the way he is avoiding our phone calls and ignoring our messages... It is very unprofessional from his side. So how can we exercise our right to the 5-year guarantee?