Hi @plavielle,

Ideally, you would use an RF scanner to scan for interfering signals,  But these are rather expensive.  You could use a tool like inSSIDER or similar from a PC or Android phone.  But this will only detect Wifi interference, not general RF interference.

Since all of your others are working, it could very well be a bad unit, band antenna, band antenna connection.  So swapping it out seems like the best course of action.  Have you tried other channels on this device?  Is it only CH11 or do all channels given an abnormally high interference.

There are a variety of sources of non-Wifi 2.4.GHz interference such as:

1) Microwave ovens

2) Older baby monitors and cordless phones

3) Some brands of CFL ballasts

4) Other non-Wifi devices that use the 2.4GHz spectrum

-Jonathan