is this adapter supports iOS 12.x and Ipad 5th generation?

I cannot confirm that. It does work on my ipad pro, version 12.3.1 6th generation.

With POE do you still need to use the charging cable?

Yes, you do. It charges the iPhone / iPad. The 2-in-1 adapter does not support POE.

Questions & Answers: iPhone iPad Ethernet Adapter with Power Charging - Lightning to RJ45 LAN Wired Network - Apple OTG USB

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Item#: 9SIA39V79A7135
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iPhone iPad Ethernet Adapter with Power Charging - Lightning to RJ45 LAN Wired Network - Apple OTG USB

Support iOS 10.1 or newer iPad iPhone with Lightning port
10M / 100M LAN adapter, stable & fast internet speed
Power charging while enjoying wired network
Accessory for game player
It's a 2-in-1 combined kit
- Apple OTG adapter -
Connect with USB keyboard, Electric piano MIDI, microphone, Sound card

$15^.99 –