How can I move/copy Domotics Pad to another device?

I have this Android SIP desk videophone that's basically a tablet with a handset. As an Android device, it's underpowered and runs Caveman Android, circa v4 BC, it sucks frankly but despite of that it makes a great control surface; it's heavy and stands firm, it's tough, it inclines forever, besides the standard PoE 2-port switch found on most SIP phones it has Wi-Fi and a power brick which is something that was when it was made a huge no-no among SIP manufacturers, they're only just coming around very recently.**

Using DPad in launcher mode I get a kiosque with Indigo's Control Pages always on and the toaster becomes useful again. Or at least it did, because a system upgrade (yet still it continued to run Caveman Android) wiped the phone and because Android continues to be old, it can't connect to the Play Store to get it again --or most sites for that matter-- anymore. I know Domotics Pad is compatible with it because it just had it, I have the app in another phone but I have no idea how to move it, for a minute there I thought I had done it using FX File Explorer but whatever I moved didn't run. I think it even wasn't an APK file either, I've already deleted it. I tried F-Droid too, I knew it was a very long shot, but I'm getting anxious to get it soon and back it up in offline storage like a root CA, before it stops being compatible.

Could you guys guide me a little? I've been an iOS user from its start until very recently, I don't have many Android devices available to use for the download or as "donor" devices if the entitlement or whatever-is-needed-to-license-the-app needs to be transferred, I don't understand how that works. One of the devices runs Android 6, the other Android 12. They're subscribed to an MDM solutions so I have that little extra bit of control, for what its worth. The devices are not rooted. I'm familiar with most major OSes' (Windows/macOS/Linux/BSDs) standard system locations but I have not an effing clue what going on there in Android. I broke the third Android device I had because I was messing with the filesystem.

Thanks!

**: On the networking side mostly, there was this device by a major vendor (also ran Android) that could use Wi-Fi for networking but it still had to take power over the ethernet port.

Hopefully we can get it working for you again... however, if it cannot connect to the Play Store that makes this infinitely harder because, having been distributed through there, it requires a connect to authorize any extended (non-base) functionality. But, one step at a time...

In order to do any side loading of the APK, you will need to be able to get into the Android settings for the SIP -- are you able to do that? What you are looking for is the the setting that allows you to install "Unknown Apps" or "Apps from Unknown Sources" or something to that affect (various manufacturers label it slightly different)

Adam

Also, if you get into the settings, look for the About section and grab the exact Android version shown there.