The 2.5G POE port is plugged on Zyxel XS1930-12HP switch. The switch supports 1G/2.5G/5G/10G, and POE++. The problem is that it is unable to negotiate with the switch at 2.5G speed. I already tried to set the switch port to auto-negotiation or force 2.5G, neither works properly. The only way to work is to set force 1G on the switch port. It seems a compatibility problem. Is it possible to set force 2.5G on AP? "set ethspeed" doesn't work.
NIC doesn't use PoE, so it isn't 100% equivalent test for switch port, and I have seen situations when one device connects on 10G on same cable, when another - doesn't, but it definitely make cable a less probable problem.
Set power mode of AP to max, and if still no success - open support cases with Ruckus and Zyxel -- as it probably will be not that simple to fix (and please post results, it is important for community to know about possible issues - multi-gig switches are becoming common).