Is a TECE Drain Line a satisfactory (regulatory) substitute for a P-Trap?

I did as suggested here. It actually didn't make any difference whether I flushed toilet/emptied basin, the draft was there both before and after the shower, so my conclusion is there's no p-trap at all for the shower or it's damaged or not operating properly. Would this be a correct assumption or is there another test (+) I can do?

I also put an amateur USB phone camera down and a pipe just goes horizontally away from the drain.

The only variance I found was the following 'insert', not sure what it's called (anyone know?), but it reduces the draft. But only I believe by narrowing the circular drain aperture, unless it has some other purpose I'm not aware of or has a functionality that makes it act as some sort of trap to prevent gas getting through.

Here's a side sectional pic of it, it was by reading off these numbers that I thought this was a TECE 'flat' DrainLine product (it's 49mm high so matches).

But I'm now wondering whether it was something that just happened to be to hand to reduce the air flow (and therefore smell) coming through, and there may not be a TECE DrainLine product below. And given new build it's taken a while for pipes to get soiled plus there's some help from a generally open door with through draft plus a ventilation system that kicks in when the light goes on.

Any ideas on next steps? Should I get someone with a camera to properly survey what's lurking down there?

#2 seal, no don't have there doesn't appear to be one, the insert they used just happens to be a TECE part, but I don't think the drain itself is, I can run a drain cleaner (flexible plastic rod with spikes on it to catch and pull debris back) in which I've done, it's clean enough as I can also see when I put the camera in.

Camera pics of below drain leading into horizontal pipe leading from there through wall to presumably sewer line (there's some debris but no blockage and it drains OK):

Thanks, yes, if that was what we're seeing here. But it looks like I assumed we've a TECE DrainLine product here (because the insert says it's TECE), when what I'm finding with the camera is that there's just a horizontal pipe connection underneath to the sewer line- NO slim trap at all. The TECE insert is just a coincidence and something the plumber had to hand to help narrow the outflow to reduce draft and smell, but it really does nothing.

In the visible drain we can see we've just a tube drop to catch debris with a 1cm ring outlet at the top, which fills then lets water outflow when you shower and air back in when the shower is off. The insert just helps narrow the ring outlet to 0.5cm so draft/smell is reduced but not eliminated.

No, no water in base of outlet. I've put the insert back in for now as it reduces the ring outlet plus all doors open for draft. And now looking for a good plumber to come check it out.

There's underfloor heating but that's no excuse. It's a new build, we've had a lot of problems. Concrete/rubble in other drains. We couldn't get hot water in the kitchen to start, had to run a new separate line to the hot water tank. The tiles in this bathroom have had to be relaid twice already as they won't stick because the concrete wasn't dried properly.

When we first started to notice problems with this drain we found the insert was filled with stones and the bathroom flooded when we ran the shower for 20 minutes by mistake. If you look at the pictures above you'll see some of the smaller stones made their way down below, now causing an obstruction that will catch hair etc…

But we'd never have believed someone would install a shower on top of the main sewer line out of the house without a decent p-trap. I still don't believe it and hope the plumber coming later this week finds something I can't see or don't know or have overlooked, I'm just an amateur here trying to work through these problems. Thanks guys for all your help.

One off build. We demolished the old house in Jan 2020, just in time to lay the foundations before C19 and then stop for lockdowns and pick up again 2 years later. Thank God C19 didn't kill us, but this nearly has…

Yes, just need to confirm what I suspect and understand exactly what's wrong before I approach the builder.