Saint AAI

Recently switched to Sky and got a bundle with TV and phone/internet. Now I know there is a way to use my own modem/router, but I didn't want to break any of Sky's T+C's. So I've decided to just plug mine into the Sky one and use it purely as a router. I started with them both running side by side and my lodgers MAC connected to the old wireless network, but dropped out occasionally. I couldn't get my phone to or the PS3 to connect to the old WiFi, but it did connect to the new Sky one. I thought the two wireless signals were probably interfering so turned off the Sky wireless. Everything connected to the old network now apart from the PS3's. I had the change the PS3 DNS settings to manual and entered in the open DNS settings. They now connect and work so I will change the DNS settings in my old router later and hopefully everything will work ok without me having to change settings on individual devices. The MAC's connection still drops occasionally.

Can any internet experts explain to me why I've had to do this? It worked just going through Sky's router, but not through my old router first without changing DNS?

Also the reason for doing all this is because my old router has dual radios and so has both n and g networks running at the same time so my friend can connect to the n without me connecting to the g messing it up, also I have a network drive plugged into it.