Following this discussion with Izzy, I think your scenario is the opposite of the described problem in that post, it seems you simply want stick to the 2.4GHz connection because it is associated with better signal strength (although slightly slower when compared to 5GHz).
As such I'd recommend this tool called, Wifi Band Switcher on this thread. This app has been reverse engineered from the code from that gives advanced wi-fi menu since it essentially uses some standard AOSP hidden APIs. So the developer created an app that uses these same hidden APIs. With this you can force the WLAN band to be always the 2.4GHz.
Note: the app must be installed as system app.
Just extract apk and using root explorer paste in system/app set permissions
Not the prettiest interface but could possibly do the job, it is essentially the same implementation found in native android ROMs:
Alternatively you could try using using Wi-fi analyzer by farproc (to analyse the available bands) in conjuction with its open source plug-in Wi-fi Connecter Library to selectively connect to the 2.4Ghz band.
If you are viewing the AP list of networks in Wi-fi Analyzer which
shows you both the 5.0Ghz and the 2.4Ghz, with this plugin installed
you can then tap and connect.
Additionally as suggested by Izzy in that discussion, you could also try
Smart WiFi Selector(trial)