Internet speed slows down when connecting to router

I have in my house a NETGEAR RAX80, there is fiber internet using a KingType PN-B04C modem, the modem was install by the cable company.  We have 200 mg od download, when i connect the RAX80 router to the modem the speed comes down to 45-55 mg, even if connected with cable to the router it still shows 45-55 mg.  The cable company suggests not using the router, only the modem they installed, the problem is that I have all the house cabled with NETGEAR equipment and repeaters.  

Again the router works perfectly, the issue is the speed, it comes down even with cable or wifi (same speed).  When using the modem installed by cable company it ready 180-195 mg, so I really do not know what to do.  They have come to my house at least 4 times and can not solve the issue, so they recomend not using our router only their modem.  I only have interet on one part of the house.

ANYONE can help with this issue PLEASE.

Enrique