Hi Tuan,
Is the AP at 27m itself or closer to the ground? What’s the transmitted power configured on the AP? What antennas?
I’d recommend to mount the AP at a lower height and/or consider directional antennas. Please bear in mind, that free space path loss might be a problem with low powers. On the other hand, significant difference between AP and MU power levels may result in very poor performance (typically client devices have lower TxP, but not always, see clients.mikealbano.com table for example). This mismatch could give lots of retries and thus increased delays and lower user throughput). I’ve not much experience with power poles to say if any other EMF effect may come into play.
I hope we can get few more details from you so we can be more precise with any advice.
Hope that helps,
Tomasz
