rovnice={
0==iU[t]+iR1[t],
iR1[t]==iC1[t],
u1[t]-u2[t]==iR1[t]*R1,
iC1[t]==u2'[t]*C1,
u[t]==u1[t],
u2[0]==0
};
u[t_]:=5*Sin[(10^4)t+2.1];
soucastky={R1->(10^3), C1->(10^(-6))};
reseni=DSolve[rovnice/.soucastky,{u1[t], u2[t], iU[t], iR1[t], iC1[t]}, t];
Plot[u2[t]/.reseni, {t,0,0.025}]

ClearAll["Global`*"]
rovnice={
0==IU+IR1,
IR1==IC1,
U1-U2==IR1*R1,
U2==IC1*(1/(I*ω*C1)),
U==U1
};
soucastky={R1->(10^3), C1->(10^(-6)), U->5*(E^(I*2.1))};
omega={ω->(10^4)};
reseni=Solve[rovnice/.soucastky, {U1,U2,IU,IR1,IC1}];
Plot[Im[U2*(E^(I*ω*t))/.reseni/.omega], {t, 0, 0.025}]

LogLogPlot[Abs[(U2/U1)/.reseni], {ω,10,(10^7)}, PlotRange->All]