Oh, I see. I read that information as saying that ##f(0,t) = U_0, \quad f(L,t) = U_1##. In this case, you seem to have mixed boundary conditions then.
##\frac{\partial u}{\partial x} (0,t) = 0, u(L,t) = U_1 ##
To use the separation of variables after your last step, you will find an appropriate...