An app stopping working on Nebula is inevitable, it is not a mainstream product from the app developer’s perspective, chances are the service owner changed something their end, and the Nebula is not updated to match, so doubt it will work again.
More things will break.
Workaround for a permanent solution is to buy an HDMI connecting something which is mainstream, something popular like a Chromecast or whatever matches your needs.
These niche products with Android are doomed, not enough sell to make it worthwhile the app owner to keep working. Just enjoy it while it works, knowing at least you got HDMI way out.
You can attempt to email Nebula support, they may know a fix. The typical fix is they know a version which works and pre-load that last known good working version, and disable play store updates as newer versions won’t work, but that only works so long as eventually the app needs updating and the projector’s DPI and resolution don’t get supported updates.
Ideally you know this at purchase time and factor into the decision, much better than assuming something sold only in the hundreds to thousands will keep working while devices sold in the millions move on.
If you’re wondering exactly what is at fault, it probably is the DPI. To support higher spec newer devices they update the backend to support newer DPI. But those newer DPI devices have more pixels than these low resolution projectors, so you find the image is either not working or you see just a corner of the image. All projectors with built-in Android are doomed, it’s not really Nebula fault but the fault of those who just assume because it works today it will keep working. A dumb projector not running Android, if it’s lower cost but otherwise works well, is a better choice. If you do insist on buying an Android projector just be grateful for what works while it works.
Further, all existing Android devices have a similar issue in that the manufacturer supplies updates only so long, then the app stops working on the older Android. Most Android devices can be unlocked to work on newer Android but the unlocking breaks DRM so encrypted content doesn’t work.
Apple does a similar thing, they stop supporting older devices, then websites or apps won’t support.
The longest living thing is Linux but eventually some content requires a hardware feature which doesn’t exist and even Linux devices become less and less workable.
Everything dies. But a niche Android devices dies fastest.
