EX7300 - using two extenders in large house

I use a rented Comcast Technicolor router (CGM4331COM) and used to use their PODsto extend the signal across 3 floors.  It worked sometimes, but the part I struggled with was the fact I coudn't manually assign devices to 2.4 or 5.0 bands.  I have a bunch of stuff, like my Blink camera system and myQ garage door unit that won't work on the 5.0gHz band.  So, I got rid of the PODS and replaced them with the EX7300 in extender mode. Works perfectly!  The EX7300 is on the same floor as the router and I need to extend the wifi signal to the basement, so i bought a second 7300.  Now, I'm not sure how to set this one up and everything I've tried isn't working.  

Should I try to connect this one to the first one or directly to the Comcast router?  Any feedback is greatly appreciated!

Thanks.

Cheryl