Premier internet dowload problems

Hello, I have Premier internet, but my download speeds do not go above 25 mbps. My upload speeds are usually between 20-25. I have a Motorola Surfboard SB6120 DOCSIS 3.0 Modem and a Asus RT-N66U dual band N router, and I called and gave them the modem's serial number and pin number to make sure that was in the system. I have connected my computer directly to the modem to check the speeds, and it never goes above 25, but the representative today said that the signals looked good going to my unit. He said it could be my modem, but I have only had it a couple of years (about 3-4). What else could I do? Buy a new modem? Otherwise, if it doesn't go above 25mbps download there is no use paying for premier at this point...