UTP (unshielded twisted pair) Ethernet cable is much more common than STP (shielded twisted pair), but you can find it pretty easily, and it is not much more expensive.
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What category of cable is it? I've heard high numbers like Cat 7 have good shielding.
Cat6 per the listing on Amazon, but I don't trust it.
Most cat6 is unshielded. Make sure you're using shielded cat6 and shielded connectors. Check it with a multimeter to make sure there is continuity from end to end too.
is the power supply or switch some bargain basement stuff? No-name chinese switched power supplies are garbage. I have an LED power supply that generates interference all the way up through 2 m.
for many POE devices, you could inject voltage from a linear power supply onto the wires. Might be an option if the device isn’t too “smart” about it’s POE protocol.
Both the switch and the WAP are Ubiquiti, a reputable brand. The AP does not use passive PoE.