Major issues after apartment renovation, builder gone awol

So I had some work done in my 2-bed apartment in Dublin, which was renovate the main bathroom and add an ensuite to the bigger bedroom. The work was finished around mid-August, but unfortunately there have been a couple of major issues. The main one is that the new ensuite shower is leaking, with water coming up through the wooden floorboards. The reason for this I'm told by the builder is that there are now two new pipes going into the main soil stack (new toilet and shower), along with 3 existing waste water pipes for the kitchen sink, washing machine, and dishwasher. It looks like the main cause is that there isn't enough of a fall towards the soil stack. I added a couple of pics to show the new pipework that was added for the ensuite.

Some other issues are that the new ensuite toilet and shower start backing up and filling with foul smelling water when the kitchen tap or dishwasher is used, and the toilet starts making bubbling noises. There is also a small leak in the main bathroom where a new shower tray and screen were added, and the washing machine that's connected to the new pipework leaks after a wash.

The last reply from the builder was over a week ago, where he said he was trying to arrange for the plumber that did the ensuite to come back and look at it. However he's now stopped answering my calls and messages, so it seems like he's gone awol.

I already paid for all the work in cash. The builder seemed to have a reputable company, but when I googled him there are a few with the same name, so I haven't found him yet. What are my options if he doesn't get back to me? All correspondence has been over WhatsApp and I didn't get a formal contract. I don't want to name and shame yet but I will if I have to, and the builder is from this parish shall we say.

Sorry to hear that you are having problems with the builder. There are some previous AAM threads on this same subject here.