I think the answer to your question is quite simple. Chances are your drainpipe is clogged and needs to be cleaned. But I doubt you can do it yourself. If there is no man to whom you can entrust the most important thing - your shower cabin, then I can tell you where to find such a man. Here https://pomplumbing.ca/snaking-and-power-wash-of-kitchen-stacks-and-clogged-drains/ you can ask for a master to be sent to you. He will see what the problem is and can fix it.