Well, finally I think I might have identified the problem.

What I did -
1. I brought another TL-7510s, and tested it, it works perfectly
2. Then I reset both units to factory settings.
3. Set up both units to Test mode, 27dBm, except the channel was different.
4. Placed both units 2m away from laptop
5. Started Acrylic WiFi, and this is the result:


AP1 (the good one) signal strength is -47, and AP2 (the problem one) is -81! That's a huge difference in favor of AP1, as you can see in the picture.

Conclusion: AP2 is defective... under the same circumstances it produces way weaker signal than the other working unit.

Now, is there some knowledgeable TP Link people who might care to comment? What might be the reason for a weak signal coming from the unit?

@Sizif I'm curious do me a favor, make this test, let's say at the factory they reversed outdoor antenna and internal 7510 has 3 setups vertical horizontal and external can you test the 3 and see in if outdoor and vertical are reverse

Sorry danymarc, I already took the unit in for repair before I saw your post. It was working well only on the first day, so I doubt somebody switched the polarization (nobody ever opened it)... what might have happened is that some of the internal antenna contacts got loose, but I didn't want to open it and check, as it is under warranty, better let them take care of it. :D

Sizif wrote

1-took the unit in for repair
2-It was working well only on the first day

1-Tp-link has 2 years guarantee
2-Like I posted, I was lazy to read it all, if it was working, you are right.