I was having the same problems with the DPC3825. Bottom line is it may be a good modem but it is a horrible router. I ended up switching back to my 10 year old Linksys router and surprise - steady uninterrupted wireless connection on all devices.
If you call Rogers, their response will probably be to reboot the modem. In the vein of what GordOh has suggested, I would further add that there is only one place to boot this modem and that is out the window! I would further suggest that there is no such thing as an "upgrade" to a Rogers D3 modem. To use a 3rd party router, which should solve any wifi signal issues, you will have to have the Cisco modem placed into bridge mode!
Well, not technically. The BEST way is to put in bridged mode, and let the router handle all routing and wireless.
The routing on these units is actually not too bad. It's usually the wireless that is the issue. By disabling the wireless on the device, and pluging in a wireless router with dhcp off, etc, you can turn the router into a wireless access point. I have had this setup with both the Cisco and smc, and does work.
But the preferred method generally, is bridged mode.