Same issue here on a Oneplus phone with Android 8.1 and up-to-date Unified Remote client and server.
Only when I open the app first the widget starts working. I am then also getting Android notifications that Unified Remote is using the battery. This started happening awhile ago. I update Android a lot so fufnf may be onto something about it being an issue with recent Android power optimizations.
Is this issue being looked into? It's pretty annoying as I use the widget a lot.