Connection worse after modem upgrade

I recently received a "Your internet equipment needs an upgrade" email.

I had an Arris SB8200, but our service was updated to 1200 Mbps, which it was not capable of.  Around this time speed tests showed we were getting less than the previous 1000 Mbps connection speed (~600-700).

I just replaced the modem with a Motorola MB8611 and now the connection is even worse, rarely surpassing 500 Mbps.

There have been no other changes with the local network and this problem is consistent even if other devices are eliminated, i.e. External coax > Modem > Laptop.

I suspect this has to do with the configuration set to the modem.  Trying to explain this to the bot is futile, so hoping to get a reasonable response here.

I can supply any additional info required.  Thanks.