Saving Battery by switching LTE to 2G when screen is off

Hi guys,
I wondered if there is a way to switch my data to 2G when screen is off and turn it back to LTE when screen is on in order to save some battery?
I've tried the following via Tasker:

status>display>off
then
plugin>secure settings>custom ROM actions>LTE>off

status>display>on
then
plugin>secure settings>custom ROM actions>LTE>on

When I toggle LTE off does it use 2G automatically and if so, how can I see if it really works? :)
I am rooted ofc...