In India for 5G, we need only two bands as of now n28 and n78. Airtel pan India uses n78 (NSA). JIO pan India uses n28 & n78 (SA). I use JIO SIM in a metro city and hence my iQOO Z3 catches the superior n78 band with 1 Gbps speeds. However ...

Can't use 5G network... I've changed the settings to 5G prefered but i get 4g network only why is that 5G not working?

You are correct, I never knew this. My iQOO Z5 came with Google dialer and by the method you mentioned enabled Vivo dialer and using it since. Super.