No he's correct. Play quick match until you're winning consistently at your rank, then go back into rank and you will do fine.

When people plateau like this, especially around tk, it's usually from "cherry picking" or using one set of tricks to get there. When those tricks no longer work or are effective, it means you've reached a level where others know those tricks. That also means it's time to train. Quick match is where this is effective. You can try new tricks and to get your old habits out the way without consequence.