I am using a SIM from RWG Mobile in my PinePhone Pro.
RWG's pricing is very keen - I am paying £1 / month for a small data bundle, which is more than sufficient for my testing. (I think the bundle may now be £2 for new customers.) They also have a free tier.
I was having problems getting data working on my PinePhone Pro, because of the APN settings.
The RWG website has a page on APNs, but this is for their data-only SIMs (and only their data-only SIMs, it seems).
The APN for their non-data SIMs (but for the data service, obviously) is:
- Name: EE Internet
- APN: everywhere
- Username: eesecure
- Password: secure
And now mobile data works!