D6220 AC1200 All 2.4 GHz channels extremely slow
Hoping someone can help me here - as the title suggests, I have a D6220 AC1200 router and the 2.4 GHz wifi speeds are horrendous. My internet connection is a solid ~45/20 mbps when connected through 5 GHz wifi channels but the speed is completely non-existent on 2.4 GHz, even though I'm practically sitting on the router.
I've tried multiple devices and setting the channels manually from 1 through 13 with very little change, sometimes I may get a little more speed initially but within 30 seconds I've dropped back down to approximately 1/3 mbps download/upload to my ISP's speed test server.
The router is brand new and I upgraded the firmware from v 18.104.22.168 to the latest 22.214.171.124 which has made no change to the network performance.
Any ideas on where to go from here?
I'm not living in a very population dense area, so i doubt i would have such extreme interference on all channels, especially since my last router worked fine.