Zyxel does publish the radiation patterns for their APs --- see the "Antenna Specification Matrix" document
here. The diagrams are a bit hard to interpret, at least for this long-out-of-practice EE. But I can confirm that the signal is stronger on the side away from the mounting hardware.
In my case, I've got the main AP ceiling-mounted in a second-floor room, and I get about the same signal level on the far side of the room below (~35ft away) as on the near side of the room above (~10ft away). Heavy wooden floor between in both cases, so that probably cancels out. I don't get theoretical max throughput from these locations, typically 720-850Mbps versus the 1200 I'd get if sitting in the same room --- but it's more than fine for what I do.
I think wall mount would be perfectly fine if it's on one end of your living space. If you want signal on the other side of the wall, it'd be a bit degraded, but by no means unusable.