@JaforSadik 

Hey

I will take a swing at answering these for you :)

I want to hang my deco m4 unit on the wall but by doing that I can't see the signal light on top so I want to turn it upside down. don't ask how I'm gonna hang it have an idea. I just wanna know that would it affect the performance?

No it shouldnt affect it at all.  The Deco are generally omni-directional antenna (broadcast in all directions) so reversing this shouldnt have any real effect.   I have seen wall mounts on amazon for these so its not uncommon!

another thing I'm using a single deco and on the app, it shows as Deco M4R and when starting up for 30-40 seconds I can see a wifi SSID " backhaul 5ghz" backhaul 2ghz" and after 30-40 seconds it goes away. is this normal?

It is indeed, thats is the Deco looking for / connecting to the other nodes for configuration and connectivity.  As you say this will disappear after a bit...   Wouldnt worry about this :)