Upgraded internet but now getting slower speeds than before

Yesterday I upgraded my speed from 600 to 1200. Before the upgrade O was averaging between 600-700 mbps, and now averaging 200-250ish. I've talked to multiple customer service reps with no luck. I've restarted the gateways several time in the 2 days and no luck. I've tested the wifi using 4 different devices all with roughly the same slower speeds. Any suggestions?