Old thread, but I've had the Network Extender for more than 2 years now, pretty familiar with it. Outside, my phone uses 4G. Inside my house it would use 3G since the 3G from the extender is much stronger than the 4G/LTE in the air *smile* (that's why I have the Network Extender in the first place, crappy cell and data signals). You don't have to switch your phone to 3G, it will just use it because it's stronger than 4G in your house due to using the Network Extender. Actually, I use WiFi in my house, so whether the phone uses 3G or 4G in my house is irrelevant, your phone will use WiFi if available. But I've noticed that when the WiFi is down and the router is still up, that my phone does use 3G. If the router is down (and hence the Network Extender is down), my phone uses 4G in the house. Weakly, but there's just barely enough 4G for it to use *smile*.
Sounds complicated...it would be better if the Network Extender had 4G/LTE, but it doesn't, so the picture is a little confusing. But it's just like being in an area that has 3G, but not 4G/LTE, your phone will just use 3G and be happy.