Minibox and Activation

I hooked up my minibox to my TV using coax rather than HDMI as my TV is old and does not have an HDMI port. The activation went well and after set up, turned the TV off. When I turned it back on, the activation was gone and had to be re-activated. Took the minibox back today to the Cox Store and they gave me a new one. Hooked it up, and same thing, loses activation after being shut off.  Anyone have any suggestions?