The fplot function is not the right tool for this job. Its documentation states that when called with one symbolic input it "plots symbolic input f over the default interval [-5 5]."
If you scroll down to the bottom of the documentation page for fplot there are a few other related functions listed. Two of the functions listed there will allow you to plot your surface (fmesh or fsurf.) fsurf when called with one symbolic input "creates a surface plot of the symbolic expression f(x,y) over the default interval [-5 5] for x and y." That sounds like what you're trying to do.
Of course, all of fplot, fmesh, and fsurf are supported not just for symbolic inputs but can accept function handles. See the examples on the documentation page for the fsurf function in MATLAB and try to adapt them to your problem.