Vodafone customers can, as of 2020 not to make more use of the 3G network. The provider provides only support for 4G networks, with 2G to fall back on.
Many Dutch people have started to make use of 4G internet, which is simply much faster than 3G. Also the range is better, although there is not yet a complete nationwide coverage. Vodafone wants to be here for 2020 change this, so that everyone in the Netherlands 4G is capable of receiving.
Smartphones also have 4G support, but nowadays that is quite normal and in 2020 probably even more the standard. If you have a device that not 4G, then you can only use the 2G network. This allows you to call and sms but not mobile internet.
On the 3G to 4G-page of Vodafone , you can check whether your smartphone is suitable for 4G. Is that not the case, then is it high time for a new one.