Why does the PoL LED stays off when the Link LED is still flashing slowly on the front panel of an ED3538T/ED3638T extender?

Answer:

The PoL LED stays off i.e., the PoL stops working entirely if the negotiation process of the VDSL link establishment between ED3538T/ED3638T and ED3538R/ED3638R extenders can not be completed within 10 minutes. To restart PoL, simply power cycle the ED3538T/ED3638T extender.