UPDATE: I bought two Waveshare USB-UART adaptors and plugged in two XBee Pro S3B radios. One is connected to the PC as base, and one is powered externally as rover. I can get them to communicate at 5.8 kbps using the Throughput utility in XCTU. The Range Test works sometimes but keeps crashing and resetting. The Spectrum Analyzer works sometimes but keeps resetting with an error message "error sending packet: for input string "5"".
Although it appears that these two radios are working, when I plug them into the simpleRTK2B boards, they don't communicate. The simpleRTK2B boards contain the ublox F9P GPS sensors, and this pair is designed to do RTK positioning. Does anyone have any experience with setting up an RTK system like this using the XBee radios?