iMac loses won't connect to internet after waking up

I bought my 71 yr old mom a mac thinking that it would be easier for her to learn to use, however, I can't even get the thing to stay connected to the internet.

I am using a linksys e1000 router hooked to a Charter motorola modem
I have a pc and a printer wired to the router
I have the iMac wireless hooked to the router
when the imac goes to sleep and wakes up, it "forgets" the ssid of the non-broadcasted private side of the router, and attempts to the guest side of the router for which the ssid is broadcasted. We go back in and re-enter the hidden ssid and info and re-connect, but every time the iMac falls asleep, it reconnects to the guest side of the router. I have even tried clicking the button that is supposed to prevent the computer from changing wireless internet connections. I am 6 hours from my parents and trying to do this over the phone, can someone help please?

Turn the SSID broadcast on for the private network. You'll save yourself a ton of headache and provided that you're using WPA/WPA2, you're not exposing it to any security risk.

Hiding the SSID offers very little to no benefit. Anyone looking to hack a WiFi network is not going to be using the OS's wireless controls - they'll use a WiFi sniffer. And regardless of whether you're broadcasting the SSID, they'll see it anyway.

If you choose not to do that, the best advice I can offer would be to adjust the priority of the network it chooses to automatically connect to in System Preferences => Network => choose WiFi (or AirPort) in the left window pane. Click Advanced. In the Preferred Networks list, make sure the hidden SSID is above the guest SSID or remove the guest SSID entirely.

Also, I moved your post over to the Networking forum, since it's not related to 'Community Suggestions and Feedback'.