Read Comma Separated String
The AVR implementation of sscanf() does not support parsing floating point numbers, thus you only get zeros out of it. You can't use the %f specifier with it. You need to convert these numbers by yourself with functions like atof() or similar.
Splitting a string into parts can be done many ways. You could use sscanf() to split up the string parts and then use something like atof() to actually convert to float. Or you use strtok() for splitting (see my answer to this question). Or you use one of the many other options, that you get, when googling for something like &quot;Arduino split string&quot; or &quot;Arduino split c string&quot;.
Note: Using the String class in Arduino is easy for beginners, but not a good practice (dynamic allocation, heap fragmentation). Look at Majenkos blog for more information about that. Since you already used a function for C-strings, I would suggest moving completely to C-strings (which are basically only an array of char).