It could be. I have no personal experience with your router, but I googled it and I saw quite a few mentions of it reducing download speeds with various configuration options enabled.

The best way to determine where the restriction might be would be to connect your pc directly to the ont and running the test. If it’s faster, it’s your router. If it’s not, replace the cable with a new cat6 cable. If that’s faster it was the cable. If it’s not, it’s an isp problem and their new modem probably isn’t going to help.

Double NAT shouldn’t impact a simple speed test that much