Stencil Buffer not working with SpriteMask

Hi all,

I'm pretty new to Unity, so please be forgiving as I have no idea what I am doing here. I have a circular sprite inside of a larger rectangle sprite, which has a SpriteMask set up to keep the circle inside the rectangle. I also have the circle (which is semi-transparent) using a custom shader I found on Reddit a couple weeks ago (I can't find it right now, if I find it I will link it here) that stops it from blending alphas with other sprites. Here it is:

 Shader "Custom/ShadowShader" {
      Properties {
          [PerRendererData] _MainTex ( "Sprite Texture", 2D ) = "white" {}
          _Color ( "Tint", Color ) = ( 1, 1, 1, 1 )
          [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
      }
      
      SubShader {
         Tags { "Queue"="Transparent"
             "IgnoreProjector" = "True"
               "RenderType" = "TransparentCutout"
               "PreviewType" = "Plane"
               "CanUseSpriteAtlas" = "True" }
          Cull Off
             Lighting Off
             ZWrite Off
              Blend SrcAlpha OneMinusSrcAlpha
            
         Pass {
            Stencil {
                 Ref 4
                 Comp NotEqual
                 Pass Replace
             }
              CGPROGRAM
 
              #pragma vertex vert
              #pragma fragment frag
              #pragma multi_compile _ PIXELSNAP_ON
              #include "UnityCG.cginc"
              
              uniform sampler2D _MainTex;
              
              struct v2f {
                  half4 pos : POSITION;
                  half2 uv : TEXCOORD0;
                  fixed4 color : COLOR;
              };
 
              fixed4 _Color;
 
              v2f vert(appdata_img v) {
                  v2f o;
                  o.pos = UnityObjectToClipPos (v.vertex);
                  half2 uv = MultiplyUV( UNITY_MATRIX_TEXTURE0, v.texcoord );
                  o.uv = uv;
                  o.color = _Color;
                  return o;
              }
 
              half4 frag (v2f i) : COLOR {
                 half4 color = tex2D(_MainTex, i.uv);
                 if (color.a == 0.0)
                     discard;
                 color = _Color;
                 return color;
 
                 
             }
 
              ENDCG
         }
 
     }
  
     Fallback off
 }

Here's where the problem arises, probably due to the stencil buffer. When I turn the SpriteMask on the rectangle off, this happens (the 2 circles are 2 different sprites): However, when I use the SpriteMask, the shader stops working: How can I fix this? Any help is appreciated!