The way your grounds are all tied together is not what I'd recommend. It appears you want the DCS Explorer only to have command control over one track and run the remainder of your tracks conventionally with the Z4000. I'd recommend wiring the Explorer to run a loop (or the whole layout) independently of the Z4000 track outputs. But this is a separate issue from getting your iPad & app to work with the Explorer.
Apple products get a bit goofy when connecting to Wi-Fi networks that do not provide internet access. I avoid Apple products due to this and other quirks that generally make them less developer-friendly. Search this forum for connecting Apple devices as many topics with happy resolutions are available to problems similar to yours.
For reference, here is a quick demo video of how the DCS explorer boots up and operates a locomotive on a test bench: