Not able to achieve Xfinity plan speeds--support says tech can come out in mid-August

In early June I signed up for Gigabit service (upgrading from Extreme 150). Installed new Arris 8200 modem. I only achieve peak speeds of ~300Mbps. Additonally, the internet disconnects multiple times a day and streaming services that historically had zero problems regularly drop and can't resolve connection. I contacted Xfinity and they apologized, but don't offer any real solutions (ie, they reset my modem and test the connection). When I complained, they created a ticket and the service date is mid-August.

Any suggestions on what issues are going on??

Thanks in advance.