Question about TP-Link Deco M5 Dualband WLAN Mesh Set of 3 (AC1300)

Here's an example of a design for 3D printing.

https://www.thingiverse.com/thing:3831...

Various suppliers from the Far East have them on offer. Most of them are vertical (possible limitation of previous comment). But there are also horizontal ones.
Search help "tp link deco m5 wall mount".

I had the same problem, you can find wall brackets 3d printed but it is a bad idea. The device needs to be horizontal because the signal is not omnidirectional. So I bought white soap dishes (search soap dishes here) and used those. They really look like they were made for it.