Bad product; no response to complaint

I ordered a waterproof dog bed on Gumtree approximately a year ago. The original delivery was efficient and I paid in cash for the product which was a large-size dog bed for outdoor use. Unfortunately the bed was only soft for a few months, after which it became more and more difficult to turn or move. I only realized how bad it was when I moved a couple of months ago and it was like lifting a deadweight ... Obviously the fabric is not waterproof as guaranteed and whatever they used as fill has become like concrete with the water that has gotten in. I sent a message last week on WhatsApp - our original mode of communication - but I have received no acknowledgment or reply even though the message has been read.