You should also be able to sync over cellular, but I would ensure that you have a strong cellular connection where you are located. Depending on how often and how much you sync, you will need to make sure that you have an adequate cellular plan that has enough data.
The setting for enabling map downloads and sync over cellular is located in the settings page for Collector under "Map Downloads and Sync". You can also enable "Push Only" under "Synchronization" as this will only push edits created by the user on that device and will not pull edits made from other colleagues.
This blog post might help: Working with Collector for ArcGIS without syncing to ArcGIS Online or Portal for ArcGIS. — Esri UK
It uses ArcMap and syncs data locally, but you might be able to use the same workflow in ArcGIS Pro and sync data to your hosted feature service.