RenderTexture Loop Effect Shader

Hope someone can point me in the right direction.

I'm doing portals with a RenderTexture, though only the first iteration works and I get a color glitch as result. I guess there is no color information, therefore the pixel isn't overdrawn. What can I do to achieve a Feedback Loop

 Shader "PortalShader"
 {
     Properties
     {
         _MainTex ("Texture", 2D) = "white" {}
     }
     SubShader
     {
         Tags { "RenderType"="Opaque" }
         LOD 100
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             
             #include "UnityCG.cginc"
 
             struct VertexInput
             {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
             };
 
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 float4 pos : SV_POSITION;
                 float4 screen : TEXCOORD1;
             };
 
             sampler2D _MainTex;
             float4 _MainTex_ST;
             
             v2f vert (VertexInput v)
             {
                 v2f o;
                 o.pos = UnityObjectToClipPos(v.vertex);
                 o.uv = v.uv;//TRANSFORM_TEX(v.uv, _MainTex);
 
                 o.screen = ComputeScreenPos(o.pos);
 
                 return o;
             }
             
             float4 frag (v2f input) : SV_Target
             {
                 // sample the texture
                 //float4 col = float4(0,0,0,1);
 
                 //col = tex2D(_MainTex, input.uv);
 
                 input.screen /= input.screen.w;
                 float4 col = tex2D(_MainTex, float2(input.screen.x, input.screen.y));
 
                 return col;
             }
             ENDCG
         }
     }
 }