I opened the stack of XBee3’s that wouldn’t connect and, low and behold, setting CP1 got almost all of them to connect. I feel like an idiot. @ckillen, thank you so much for that catch. ModularSensors will be changed. I feel really, really dumb.
Interestingly, despite the face that I’m pretty sure there’s better service with Verizon here than AT&T, all of the modules chose to connect to AT&T.