I can't ping...if i use short cable about 8meter....the lan indicator stable..if i use 15 meter...the lan indicator did't light

Model : TL-WA5210G Hardware Version : V2 Firmware Version : ISP : hello.. i use tl-wa5210g for AP to transmit data from point A to B..at point B use same model...but my problem is POE from point A i...