Correct. They are using some proprietary settings in their LoRa. Many of us wish this were not the case.
Yes you can use the Lemo connector RS232 to connect another matching radio. I have done this with SiK 3DR radios and RFD900 radios. There is a document describing how to do so for the M2, I am not sure if they have one on the RS2. Hardware integration | RTK Modules
I haven’t messed with the XBee’s. I looked at them but didn’t find any in an enclosure. This is why I went with the 3DR radios. A pair in enclosures can be had for under $40. They also have a micro USB port that can be used with the RS2’s USB port and a OTG USB cable to connect the radio that way.
I actually used my M2 with a RFD900+ to make a base station for my Trimble system. This transmits to a 3DR radio which is connected to the Trimble monitor. Both the 3DR and RFD radios work off of SiK software/principals so they do talk with each other. Here is a link to my project. My M2 project, Trimble Base Station