Keep in mind, this is what i interpret as the easiest method.

Calculating The FOV of the model. this may seem confusing.

NEW TOPICS

Vector3.Unit CFrame.LookVector

UNIT

A vector represents a direction and magnitude (length).

You can think of a vector as an arrow pointing in a certain direction that also has a length, in this case 2 units.

A unit vector is a vector that has a direction and a length of 1 unit, meaning that it only indicates direction

LookVector.

LookVector is a property of CFrames aka Coordinate Frames that represent the unit vector of the CFrame direction. If you’d like to construct your own CFrames with lookvectors, you can call CFrame.fromMatrix.

CFrame.fromMatrix ( Vector3 14 pos, Vector3 14 vX, Vector3 14 vY, Vector3 14 vZ )

Creates a CFrame from a translation and the columns of a rotation matrix. If vz is excluded, the third column is calculated as [vx:Cross(vy).Unit].

This example uses CFrame.fromMatrix() to create a CFrame located at eye with it’s lookVector pointing towards the target position.

function lookAt(target, eye)
    local forwardVector = (eye - target).Unit
    local upVector = Vector3.new(0, 1, 0)
    -- You have to remember the right hand rule or google search to get this right
    local rightVector = forwardVector:Cross(upVector)
    local upVector2 = rightVector:Cross(forwardVector)
    return CFrame.fromMatrix(eye, rightVector, upVector2)
end

A more simple explanation;

Look/Up/Right Vectors describe the direction of the Front/Top/Right faces.

Your script

Make a Local Script in StarterCharacterScripts Give the model a primarypart.

local Player = game.Players.LocalPlayer
local RunService = game:GetService("RunService")

local ModelName = "" -- name of your model
local Model = workspace[ModelName]

wait(3)

local Character = Player.Character
local found = false


local TweenService = game:GetService("TweenService")

function checkDist(part1, part2)
    return (part1.Position - part2.Position).Magnitude

end

function getFOV()
        local modelToCharacter = (Character.Head.Position - Model.PrimaryPart.Position).Unit
        local modelLook =  Model.PrimaryPart.CFrame.LookVector

        local dotProduct = modelToCharacter:Dot(modelLook)

        return dotProduct
    end

function getVisible(x, x0)
    local newinf = TweenInfo.new(1, Enum.EasingStyle.Sine, Enum.EasingDirection.Out, 0, false, 0.5)
    local visibleTween = TweenService:Create(x, newinf, {Transparency = x0})

    return visibleTween
end

RunService.RenderStepped:Connect(function()
    local modelFOV = getFOV()

    if modelFOV > .5 and checkDist(Character.HumanoidRootPart, Model.PrimaryPart) <= 50 then
        if not found then
            found = true

            print("Player is in FOV")
            local tween = getVisible(Model.PrimaryPart, 0)
            tween:Play()
        end
    else
        if found then
            found = false

            print("player is not in fov")

            local Tweenzz = getVisible(Model.PrimaryPart, 1)
            Tweenzz:Play()

        end
    end
end)