No issue staying on 5g on mine and I have 2, 2.4ghz channels and 2, 5ghz channels. I don't even put in the key to login to my 2.4ghz channels, that may help you as well because then your note 5 won't have another ch to switch to. I'm using a netgear nighthawk router connected to a generic Comcast Cable modem/router combo. All is well.
Posted from my Beast Galaxy Note 5
DEVICE HISTORY (since they got smart)
PHONES: Galaxy Nexus, LG viper, Galaxy s3, HTC Evo 4g, Galaxy Note 2, Galaxy s5, LG G-Flex, Galaxy Mega 6.3, Galaxy s6 edge, Galaxy Note 5. TABLETS: Galaxy Note 10.1 gt-n8013, Galaxy tab 3 7.0. WATCH: Galaxy gear