Really not sure what you are trying to accomplish with the dynamic dns service. I dont think it does what you think it does.
Dynamic DNS = makes your home dhcp address accessible from a 'static' dns entry even if your home IP changes. IE: myhomecomputer.dyndns.org will always point to whatever IP address you have at home, even if it changes due to dhcp.
18.104.22.168 is google's public DNS server, though I have found it to be a bit hit or miss in the past. I tend to use MIT as my primary (22.214.171.124) with google as my secondary on machines that I dont have my own internal DNS server running.
Honestly, at this point I would start to assume you have something wrong internally. You RMA'd that one router, but maybe that model/brand just wont work well with your service? Maybe you have some kind of wiring issue happening? You keep trying to beat a dead horse on this whole thing and I think it is just causing you more headaches. Beg/borrow/steal a router/firewall from someone and see if it has the same problems. Hell, try connecting a PC directly to your cable modem and see if you get disconnects. At this point, i dont see any other options, you need to do some actual troubleshooting to see where the problem is instead of just guessing and assuming you know where it is.
Cut your problem into chunks:
1. Eliminate everything behind the modem, connect a pc directly to the modem. If that still has the issue, try another PC. Still there, its the modem. (EDIT: or something you have configured on your machines, try to use as default of a machine as possible with automatic everything for network settings)
2. if no disconnects above, beg/borrow/steal a firewall from someone. Hook it up with the factory config and the absolute minimum of settings (leave the default LAN IP range for now). Hook up the same PC as in step 1. Dont assume if this works that it is the modem, you still only have a single machine on it.
3. Hook up rest of machines to this device, keeping the same config from step 2.
4. Still working, configure the new router/firewall as you would normally.
5. still working? suck it up and get rid of the rosewill. Well that or accept that the problem is that unit, and no amount of messing with it will fix the problem and move on.
Sorry to be a bit harsh here, but this has been going on entirely too long for a very simple thing. This is pretty basic troubleshooting here. If you cant beg/borrow/steal from someone you know, post here where you are roughly and see if anyone will be nice enough to ship you an old unit of theirs (hell i may have one laying around somewhere).