i mean 4G advance (4g+) not Vo-Lte (voice over lte)

4G+ is dependent on your network. It's available by default, but it matters which region you are and the network you use. It could be due to network congestion, or your network doesn't use all the bands used in the device. If you are in Europe, know that POCO doesn't support all the network bands, so even if your network supports 4G+, your device may not support it. Here in India, it is available on regions where the network is strong enough to support it.

Cheers! :-)