Updated speed but connectivity is worse

Xfinity recently updated my internet speed to 400 mbps. Ever since my internet has been slow and cutting out. I’ve restarted my Arris SBG8300 multiple times but it doesn’t help. 

I made an appointment for someone to come out but Xfinity cancelled it and said they did work in my area that should fix the issue. It did not. Should I make another appointment or try something else? Thanks!