could be issues with the firmware in the phone. its a new product and there are bugs for sure.
i use a cisco e4200 and run mixed mode. the one uses 5ghz here and at the job it uses 2.4. no issues at all at either site. i am stock and not rooted.
not sure if a new router would help but i can assure you i have no issues with my setup. i have several mixed mode devices and they are all playing nicely.
i have seen a post or two about this issue you are seeing. its the weekend, so give it a day or so when others log on and check this space. you may have some others who have the issue.
although i ALWAYS run "best wifi performance", some who initially did not run this setting ticked and then DID noticed things improved.
also make sure fastboot is NOT ticked....
EDIT: so i dont use any battery saver modes, do you by chance have your phone set to kill wifi at idle or sleep? this may be confusing the router if you have the phone set to shut off wifi when idle, etc..