Xfinity email about equipment replacement

I received an email about replacing my old owned modem to get faster internet speeds. I bought a new Netgerar CM 1000v2 modem and activated it online. My speeds increased from 150mbs to 400mbs for two days then went back to my old speeds. Contact with 3 customer service reps was no help. They all say my service plan "Performance Starter" is only rated for 150mbs. However my latest bill states "Download as fast as 400mbs", and the email stated " These faster speeds are included with your current internet plan at no extra cost." How do I get the faster speeds at no increase in price?