I temporarily use *#*#4636#*#* to force my phone to EDGE/GSM only. I can get 4-5 days on the battery and can stay on top of my email/whatsapp with light data use that doesn't require high speed. It's great when I'm traveling/vacation. Problem is this setting doesn't stick. If I restart the phone, it switches back to 4g LTE and if I forget to toggle to 2G mode, it drains the battery.
Any permanent solution to address this?