Texture offset not working with shader

As soon as I added a shader from the asset store onto a quad object in my game, I was unable to use renderer.material.mainTextureOffset or the offset slider in the inspector to shift the texture of the material on the quad. I have little experience with shaders, so I was hoping someone would take a look at this code to confirm my suspicions that the shader is not allowing texture offset:

 Shader "Black Hole" {
     Properties {
         _MainTex    ("Main", 2D) = "white" {}
         _Center     ("Center", Vector) = (0.5, 0.5, 0, 0)
         _Distortion ("Distortion", Float) = -2
         _DarkRange  ("Dark Range", Float) = 0.1
         _Warp       ("Warp", Float) = 30
     }
     CGINCLUDE
         #include "UnityCG.cginc"
         sampler2D _MainTex;
         float2 _Center;
         float _Distortion, _DarkRange, _Warp;
         float4 frag (v2f_img input) : SV_TARGET
         {
             float2 uv = input.uv;
             float2 center = _Center * _ScreenParams.xy;
             float dist = distance(center, uv * _ScreenParams.xy);
 
             float2 warp = normalize(_Center - uv) * pow(dist, _Distortion) * _Warp;
             warp.y = -warp.y;
             uv = uv + warp;
 
             float light = saturate(_DarkRange * dist - 1.5);
             return tex2D(_MainTex, uv) * light;
         }
     ENDCG
     SubShader {
         ZTest Off Cull Off ZWrite Off Blend Off Fog { Mode Off }
         Pass {
             CGPROGRAM
             #pragma vertex vert_img
             #pragma fragment frag
             ENDCG
         }
     }
     FallBack Off
 }

I imagine there is an easy explanation for this, but I must have missed it during my searching through forums and documentation. Any help on this issue would be much appreciated.