There are a few posts floating around about this topic and there does not seem to be any official comment from Telstra or Samsung except that a Note 10+ 4G bought from Samsung and used on the Telstra network will not use Telstra RCS. This has been confirm on other posts to be fixed by forcing the device CSC to TEL (Telstra) using paid tools.
From my research the variant sold by Samsung directly is a Multi-CSC version and therefore should automatically change to the carrier CSC on first boot based on the SIM inserted. On my device the available CSC list includes the Telstra CSC (TEL) but does not get automatically selected when the phone is factory reset and booted with a Telstra SIM.
Wouldn't this mean that the problem is with the SIM card or carrier not the phone?