By design, AP Isolation is meant to be a network-level block, a firewall of sorts, preventing one device on the local network (LAN) from talking to another. This is fairly standard on public Wi-Fi hotspots.
There are two workarounds:
- If you have a smartphone that can set up a Wi-Fi network, you can use that as your W-Fi access point and router. You won't get dinged on cellular data bandwidth, because all traffic will be inside the LAN. It has the advantage of allowing Internet connectivity for Mac/PC and iPad if needed.
- You can create an Ad-Hoc Wi-Fi network between your iPad and Mac/PC running Lightroom. The downside of this is you won't have Internet connectivity, but at least sync will work. The procedure for setting up an Ad-Hoc Wi-Fi network varies depending on if you're Mac or PC.