Internet now much slower after "upgrade"

I pay for 300Mbps service.  I've measured a little over that recently, which was great.

Then I got an email that they were "upgrading" me to 400Mbps for the same cost.  I just needed to restart my modem.  So I did.  I now measure less than 95Mbps at 1:00 a.m.  And even slower during the day.  How do I get back the speed I'm supposed to have?