Shader for Portal

I'm trying to make portals like in game, i found good shader? but i have problem. Portals render itself in another very bad. So, how can i fix it? Here is the shader

 Shader "Unlit/Portal"
 {
     Properties
     {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         [HideInInspector] _PortalTex ("", 2D) = "white" {}
     }
     SubShader
     {
         Tags { "RenderType"="Opaque" }
         LOD 100
  
         Pass {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 float4 port : TEXCOORD1;
                 float4 pos : SV_POSITION;
             };
             float4 _MainTex_ST;
             v2f vert(float4 pos : POSITION, float2 uv : TEXCOORD0)
             {
                 v2f o;
                 o.pos = mul (UNITY_MATRIX_MVP, pos);
                 o.uv = TRANSFORM_TEX(uv, _MainTex);
                 o.port = ComputeScreenPos (o.pos);
                 return o;
             }
             sampler2D _MainTex;
             sampler2D _PortalTex;
             fixed4 frag(v2f i) : SV_Target
             {
             fixed4 tex = tex2D(_MainTex, i.uv);
 
             fixed4 port = tex2Dproj(_PortalTex, UNITY_PROJ_COORD(i.port));
 
             return tex * port;
             }
             ENDCG
         }
     }
 }