Change Router MAC, get new WAN IP without Cable Modem Reboot?

I had an old Barricade router that I was using with my current cable modem, and it was able to reconnect to my cable modem after I modified the MAC it reports to the cable modem, but both the Pepwave (and the Cradlepoint I had before that) lose their connection to the cable modem until I reboot it.

Does anyone know what special magic the barricade did, or how to avoid having to reboot the cable modem after changing the MAC? At one point I was going to sniff the traffic with a hub to figure out what the magic was, but that never panned out.

To repeat, I know my cable modem is capable of doing this, because it worked with that old router, but for some reason the newer routers aren’t as smart.