If your phone is 4G you must choose the option called LTE / WCDMA / GSM (Automatic connection) which means that once we do not have 4G coverage it will automatically connect to a 3G or 2G network depending on the coverage that exists.
But if your SIM card or plan is only 3G the option we need to choose is WCDMA / GSM that works with 2G and 3G, with this you will not lose the signal anymore and your device will not be out of service.