You are aware that the Motorola modem is a 1000Mb WAN port speed supporting right?
I presume there could be an issue with 605L.
If this unit is new and returnable I resommend returning it and getting a DIR series model router that has Gb supporting speed on the WAN port. DIR series models that support this speed is DIR-636L and above. All DIR 8 series routers support Gb speeds.
The Motorola is a good modem and I would not recommend installing a bottleneck between you and the modem.
Also be aware, If the ISP modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems: Link>
Double NAT and
How NAT Works. To tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged. If the modem can't be bridged then see if the modem has a DMZ option and input the IP address the router gets from the modem and put that into the modems DMZ.