What controls the switching between 4g and 3g?

I use a Sprint Note4 rooted On OBAR ROM. While at work, my service switches from 4g to 3g very often. The only way i can get it to switch back after it goes to 3g is to either turn off data or turn on airplane mode and off again. It is very annoying. I also use the gtools program and lock it on LTE. It works until i guess i lose they lte signal. Then i lose all connection. So i change the setting ball to lte/Gsm/cdma.
Any and all suggestions are very well appreciated.
Blessings