My god, you’ve just reminded me of our absolute nightmare scenario that happened when we got the the first of our two apartments opened up.
First Airbnb guest, lovely guy working at the US base in Rota, staying for five days.
Got a message from him after first night, “have you got a plunger, I think I’ve blocked the loo”.
Told him where it was and checked all was good. Yes he says.
Same story at check out, so we assumed he’d simply used too much paper, or had been eating too much red meat 
Next guest a one nighter from BDC the following day. Couple of strange German women who sold BitCoin for a living.
Inspecting apartment after check out found several floaters 
Tried flushing, no go. Used drain rod, liquid unblocking solutions, slight improvement but still not flushing like it should. Got guests coming the next day and second apartment nowhere near finished. Spoke to builder, he says someone must have flushed a nappy or something down it.
This then became a circular discussion where he says nowt to do with him and suggested we get the Dyno Rod guy out. No way, €400 minimum!
My OH was talking about cancelling next guests, which would have been disastrous to our standings. So, I started to think about it logically. I knew it was a new waste pipe, I’d been there when it was being fitted, and it connected to the main waste which was flowing (and flushing to) fine.
Only thing left is the actual lavvy. Out with the tools time.
When I removed the pan the problem was obvious, the guy who’d fitted it had used a straight connector from pan to waste, when he should have used an offset one. It meant the joint was compressed and only allowed small poos to pass, so to speak.
Quick run to hardware shop, €5 and twenty minutes work later, a perfectly flushing lavvy. I even road tested it myself 
It’s funny when we look back on it now, but at the time it was shaping up to be not the best start to a STR business! Both sets of guests gave us good reviews despite the problem, they probably blamed themselves. Our builder was very quiet after I told him, especially as I’d photographed the evidence, both human and hardware related 
JF