TP-link Extender fixes the xfinity / Arris SBG10 Issues????

Recently bought a Arris Surfboard SBG10.  As soon as I installed it, I would have intermittent drops througout the house.  My signal strenght in the kitchen would be maybe 2 bars on my laptop.   I had ZERO issues with my other wifi router(linksys) other than it was older.    

Arris tells me.....  after a technician looked at my power....  said i needed to calll xfinity..  because my 

'Cable signal levels are outside of the ranges for UPSTREAM' 

Yes..  I called xfinity, they say 'its all green and everything is fine'

I have read all the threads about this.   

then I thought...  hey.  Let me try an extender'  sd I just installed a simple tp-link AC 750 dual band.     put it CLOSE to my arris Wifi router.   And

IT WORKS!     No  low bar in my kitchen.   No drops.

Am I wrong or does this tell me it IS THE ARRIS Surboard that is the issue?