This is what I've tested
local cframe= CFrame.new(model.PrimaryPart.Position) -- CFrame with default rotation model:SetPrimaryPartCFrame(cframe * CFrame.new(0, math.rad(90), 0))
and this
local Model = workspace.Model -- Replace this with your model. Model:SetPrimaryPartCFrame(CFrame.new(0, math.deg(90), 0))
Both of them rotated my model 180 degrees and it did even if I put somthing else instead of 90
The problem is that you are setting a new CFrame and not using the current CFrame of the model, you have also use CFrame.new instead of CFrame.Angles. You also need to be using radians with CFrame.Angles.
local Model = workspace.Model -- using the current CFrame rotate the CFrame by 90 degrees Model:SetPrimaryPartCFrame(Model:GetPrimaryPartCFrame() * CFrame.Angles(0, math.rad(90), 0))
I hope this helps.