Hi Andrew,
Welcome to the Emlid community forum!
LoRa radios from different manufacturers may be partially incompatible due to the different firmware and configuration protocols. That’s why it may not be possible to establish a stable RTK link between them.
To work on longer baselines, I recommend using an external radio for communication between Reach RS2 units. The main requirement is the ability to allow Serial communication.
We have a post about testing Reach RS+ with external UHF radios: Reach RS+ tests with external UHF radio (450 MHz). This radio also works fine with Reach RS2. I believe it can be helpful for you.