To find address by specified coordinates, use the request get_locations. As an example, let's detect the address of the point with coordinates 27.516643322771575 EL and 53.91069937030936 NL.
<your_wialon_pro_address>/ajax.html?svc=gis/get_locations& params={ "points":[{ "x":27.516643322771575, "y":53.91069937030936 }] }&ssid=<your_sid>
Response:
{ "locations": ["Skryganova st. 6, Minsk"] }
Input data of this command is an array of coordinates, it gives us an opportunity to get location of many point in one request.