Dns server location

I am using the app to use the dns provided by nextdns. Almost everything is ok but I am really confused about your dns server location, it is showed on the frontweb that you have some servers more nearby which can surely provide more smooth experience, but I am keeping using the server which is located in the UK. while I am using the network in Wuhan, where is showed having a server here. So why can't I switch to this server? Thanks!