You do not know my qualifications.. please refrain from commenting on them.
You are correct, in general OVERALL, using the device as a modem, and using your own 3rd party router, is the best setup for most issues.. and does solve MANY problems.
I am at home, using a MIX of things, running off of the gateway itself.
I have 3 PCS set up with manual IPs and google DNS servers (2 wired 1 wireless). 2 printers, pointing at the proxy address iteself (so using rogers). 2 other devices manually set up, one with google on with the proxy address.
there are 4 phones, 3 tablets, a smart TV, a 360, and a laptop, all wireless via DHCP.
ALL connect fine, at the same time, and generally dont have issues connecting, nor browsing anything.
(other than the ocassional rogers DNS server issue.. where its slow/lagy/down.. why its on devices that dont use it quite as much)
But you are able to by pass the rogers DNS servers by OTHER means, and even by default, your own router connected, may get pushed the same rogers DNS servers. You can obviously change these on the 3rd party router.
But give it a try on with the modem setup.. most routers support it.. you can set up a manual connection with your routers IP as the DNS.. doing the same thing.
Have a good day.