What is the AP version? Are you using this AP as standalone or you have a controller managing it.
Smart Select refers to the algorithm which is used for dynamic channel assignment and as per my understanding it should be enabled by default.
Check the status if it is enabled or you have selected manual channels.
Log-in to AP GUI >>Configuration>>2.4 GHZ>>Common>>Channel >>Select any other channel (Non overlapping channel recommended).
About 5 G WLAN disappearing, login to AP CLI and run below command. In the output, check and make sure 5 G WLAN is showing in the list as up/up.
If 5 G WLAN is still showing same problem then do a factory reset and reconfigure AP (assuming you have one AP and using it as standalone AP).
To avoid congested channels on 2.4 G, prefer channel 1,6 and 11 only or you can also use a WiFi Analyzer application (Free applications are available for Android/iOS and windows) to check what channels are free and less congested (for 2.4 and 5 G both) and accordingly choose the channels.
Syamantak Omer | Community moderator | Staff TSE | CWNA | CCNA | RASZA | RICXI | Follow me at https://linkedin.com/in/syamantak-omer-5949b6124