Tasker/Security Settings 2G switch not working on OnePlus X CM13
So here is the way I solved it: After discovering the right place to edit the preferred network mode I tested to change it:

adb shell settings put global preferred_network_mode 1
adb shell settings put global preferred_network_mode1 1
adb shell settings put global preferred_network_mode2456921 1
adb shell stop ril-daemon
adb shell start ril-daemon


The problem was that the changes were not reflected and with the restart of the ril-daemon they were reset (preferred_network_mode 9).

So I used Gravity Box (Xposed module) to change it and with this it's working fine:

Send Intent [
Action: gravitybox.intent.action.CHANGE_NETWORK_TYPE
Extra: networkType:1
]


Sadly it was not working without it (so I could have saved the money for Security Settings but however.

So if you want to try it with tasker I would recommend to use it with Xposed and Gravity - that's probably the easiest way actually.