FWIW, I recently had the same issue on an RB1100AH.
The fix was to boot it using the "Boot" port on the RB (Port 13 here), and when it got to "waiting for installation server" move the cable to another port (Port 1).
Discovered purely by happenstance, noticed that Etherboot was listing ALL the ports MAC addresses so I figured it must've been watching all the ports, moved the cable over and was about to reboot the RB when lo and behold it started installing.
Did an idiot check to confirm that I wasn't delusional but the next time the behaviour was the same.
Just for laughs I just did the same thing in Windows (previously when I tried the RB would netboot but would never appear in the netinstall UI, when I moved the cable to port 1 it popped right up).
I'm actually wondering if this is an etherboot bug... Because as you say, most devices use port1 whereas the RB1100AH uses Port 13 and your CCR uses Port12, maybe after the initial netboot it's trying to use port 1 rather than the port it booted from...
Just did some further testing, and it seems like ONLY port 1 works after the "waiting for installation server" phase so I assume the bootstrap image netinstall sends over assumes Port 1 for all platforms, which is why you never see the broadcast to UDP/500 otherwise.