The Server field in the APN configuration specifies the address of a WAP Gateway server.

This field should usually be left blank.

A WAP gateway was a feature used in early cellphones and other mobile devices that were incapable of accessing normal websites.  WAP sites formed an alternative to a web site, useful for low powered mobile devices incapable of rendering a modern web page of the time. Major sites at the time supported WAP, but support is now all but irrelevant, and is not used by Android or any other modern smartphone which incorporate fully functional web browsers.

Note that this setting is not used for MMS communication, which depends on the MMSC, and in some cases the MMS proxy / port fields.