Well, the problem is solved... and the first half of it was a real stupid one. Setting the second router to not do DHCP was correct, but I had the cable to the primary router plugged into the wrong place... I mistakenly assumed
(from using ethernet switches at work
) that the "WAN" port on the second router was the same as the "uplink" port on a switch
(it's not
). Once I moved the network cable to the correct port, the HPC can see the network shares, and some other flaky network problems
(not HPC related, which is why I started digging again
) also went away.
However, I still couldn't sync. I've set up a partnership with the slow serial cable, then configured Activesync on the PC to use the network. On the HPC, I start activesync, select "network connection" and the computer name, and get "Connecting to Desktop... PC contacted. Locating connection agent. Please wait...". So I wait, and wait, and wait... nothing. Tried it again with the PC's firewall turned off, and whaddya know... it connects right away. So
a bit more digging, and according to that site I unblocked ports 990, 999, 5678, and 5679, turned the firewall back on again... and I'm in business.