Are you able to view the Synced images on Lightroom web?
Have you looked in the Folders panel? Mine are listed as Samsung or iPhone and you need to open each tab to see “imported photos”
You can create a collection from the folder contents in the usual manner.
Just to be clear, this is ONLY when creating a collection. I don't actually upload any photos from my phone to LR.
So what I'm doing is - Open LR web/mobile>Create New Collection>Move Photos From One Collection to New Collection
Sent via mobile thingy.
That worked for me except that my Lightroom Mobile created collection was in the Desktop collections panel in a collection set called "From Lr mobile".