I just bought an Archer AX3000 router. I have aSamsung Galaxy S10+ and the performance of the 2.4GHz is very slow, but only on the phone. Other devices on the 2.4G are much faster. I have ATT Fiber with a Gigabit connection, and a speedtest ont the S10 right next to the router is around 10 Mbps upload and up to 40Mbps upload.
I saw there were  some issues with S10 that were supposed to be fixed in a firmware update earlier this year. I also followed these steps mentioned in Samsung S10 series phone can’t reach full upload speed after updating Android 10 OS but it didn't make a difference. I also deleted the tether app as was suggested on another post, this didn't make a difference. Finally after I removed the ax from the wireless mode, the speed is much faster, about 100Mbps. Are there some other known issues with Samsung phones?

The 5G network has the ax mode enabled and there seem to be no issues, speed's are over 500Mbps.