Switched from MB8611 to Coda56 Modem - Experiencing Slower DL Speeds - Modem Provisioning Issue?

Hello, I recently installed a new Coda56 modem (previously using a Motorola MB8611) and connected to my Asus AX86U router. Everything is set up exactly the same as it was except the modem and one setting on the router- I enabled using the 2.5gb port to use for incoming WAN. As of today I also updated my router firmware.

I am measuring the DL/UL speeds via the router portal. Since the router is directly connected to the modem via cat6 cable, I would expect to see close to my advertised plan speed. I am currently on a 1Gbps plan and ever since swapping the modem, I have been getting 300-500Mbps download speeds, whereas I was getting 700-900 with the MB8611. The upload speed actually improved-- it went from 20-30 to 60-70Mbps.

I am wondering if this is a modem provisioning/firmware issue- I'm open to suggestions on things to check/try.