This may help others.

I have another router bridged to the Archer C2300 (AC2300) to access another device in my basement. Since I have WDS bridging enabled on the 2.4 GHz band it is unavailable. When I unclicked WDS bridging, saved it, and rebooted my router, the 2.4 GHz spectrum was available to set up my thermostat. I set my my 2.4G and 5G SSIDs and passwords to be the same so the device would recognize the network name and log on with the correct password. I unclicked the 5G radio and saved it.The network was available, but running on the 2.4G spectrum so I was able to set up my thermostat.I verified it was 2.4G in Windows wireless properties. After it was set up, I re-clicked the 5G radio,saved it and clicked WDS bridging, saved it and rebooted my router. I didn't change anything else. Everything is working in 5G as it should.*jinx*

So if you are not using WDS bridging, all you should have to do is disable the 5G radio and you could set up your smart device on the 2.4G spectrum. Then renable it after you are done. Don't forget to reboot your router.