You should improve your understanding of heat transfer basics.
Convective Heat Transfer > q = h(T - Tb)
where q = heat flux
h = film coefficient
T = surface temperature
Tb = temperature of bulk fluid on surface.
Do you see that convective heat flux depends on the surface temperature T?
A heat flux boundary condition means the value of q is a constant, regardless of the surface temperature T.
It is mathematically impossible to have both a convection boundary condition and a heat flux boundary condition on the same surface.
If you know the film coefficient, h, you could use Steady State Thermal analysis instead of a Fluent Model to simulate the convective heat transfer.