Today I tested … Telstra
Country: Australia
Provider: Telstra
Modem: BM818-T1
Calls: in and out, OK
SMS: in and out, OK
Data: OK (slow)
SIM already activated in another phone. SIM moved temporarily into the Librem 5.
Various APNs offered but this time I was smart enough to check the APN on the original phone before removing the SIM, so I knew to choose telstra.internet, which was one of the APNs offered.
If only I knew how to see which LTE band is being used for data. However it should be using B28 (the band that is missing on the -E1 and -A1 variants).