Some routers with "N" do not wrok well with prontos. I suggest you set up an extra WIFI AP with only B/G capacity as the pronto connection. Run that one with WEP128, combined with MAC adress filtering, Set the subnet mask to only let in one connection (255.255.255.254) and you have reasonable safety.
If you set up your old router as an access point only and let the new one handle DHCP and all that and use old router as the connection point for the pronto, hooked up to one of the switch outputs on the new router it will work.
The two routers should have different SSID and work on channels at least 3 steps apart.
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