Interal Network Security With Dual Router Configuration?

I was a bit of a loss and maybe it's because I have been out of the game for a long time but I came across a disturbing development. I call century link because my router was not connecting at the proper speed of 7 mbps. That problem was easily remedied over the phone but the lady said she could see several devices connected. This lost me a bit because I have a double router configuration. The Actiontech PK5000 i turned into nothing more that an internet modem. I disabled the DHCP and configured my Linksys E1200 for all the internal DHCP so there is only one cable going to the PK5000. I should technically speaking look like one user on there network with natural protection from the second router. I may be kidding myself or man I'm getting old but if someone can tell me how that works it would be fun to know and what would even be funner to know is how to stop it