I need a compatible range extender to eliminate wifi dead spots

I am a new Comcast Business internet user. The wifi router the Comcast contractor installed appears to be a Cisco devise. I find no model number on it but the s/n is 277111574.  My internet service has both 2.4 and 5.0 bands. What range extender make and model do you recommend that would be compatible with the router I am using? The contractor didn't leave a manual or anything for my router, so I know nothing about it or what it is capable of.