Upload speed is slow

I swapped out my Gigabit modem ($15/month) for a Motorola 7621 I bought on Amazon, the download speed is about the same (greater than 600mbps) but the upload speed went from 40mbps to 5mbps. I tried a second modem Netgear CM500 and get very simiarl results, no improvement - still about only 5Mbps upload. I'm still subscribed to the gigabit plan. Any idea how to get back to 40Mbps upload without renting a modem from Xi]finify??