Update

app/Models/UserPlan.php

@@ -40,4 +40,14 @@ class UserPlan extends Model
         'cancel_at',
         'canceled_at',
     ];
+
+    public function user()
+    {
+        return $this->belongsTo(User::class, 'user_id');
+    }
+
+    public function plan()
+    {
+        return $this->belongsTo(Plan::class, 'plan_id');
+    }
 }