Hi Joe,
Welcome to our community 
LoRa radio performance highly depends on the environmental conditions, so the maximum baseline varies from one area to another. It’s hard to predict the exact value without local tests. Usually, it’s up to 8 km, but some of our users reached 40 km and 50 km baselines with the built-in LoRa.
If there’s commonly no line of sight between the base and rover in your sites, it can affect the signal transmission badly. In this case, you can try out other options - connect the units via our free Emlid NTRIP Caster or a pair of external radios.