public class Tag : RealmObject
{
    [PrimaryKey]
    public int Uid { get; set; }
    [Required]
    public string Name { get; set; }
}

using Realms;
using System.IO;
using System.Linq;
using UnityEngine;

public class RealmExample : MonoBehaviour
{
    private static Realm realm;

    private void Awake()
    {
        RealmConfiguration config = new(Path.Combine(Application.dataPath, "database.realm"))
        {
            IsReadOnly = true,
            Schema = new[]
            {
                typeof(Tag),
            },
            SchemaVersion = 11ul
        };
        realm = Realm.GetInstance(config);
    }

    [ContextMenu("QueryTags")]
    public void QueryTags()
    {
        string substring = "T";
        IQueryable<Tag> tags = realm.All<Tag>()
                                    // The line below is the one causing the exception
                                    .Where(t => t.Name.Contains(substring, System.StringComparison.OrdinalIgnoreCase));

        foreach (var t in tags)
        {
            print(t.Name);
        }
    }
}

NotSupportedException: The left-hand side of the Call operator must be a direct access to a persisted property in Realm.
Unable to process 'value(RealmExample+<>c__DisplayClass2_0).substring'.
Realms.RealmResultsVisitor.GetColumnName (System.Linq.Expressions.MemberExpression memberExpression, System.Nullable`1[T] parentType) (at <5ee71c2d3038455ca732a3febcca3879>:0)
Realms.RealmResultsVisitor.VisitMethodCall (System.Linq.Expressions.MethodCallExpression node) (at <5ee71c2d3038455ca732a3febcca3879>:0)
System.Linq.Expressions.MethodCallExpression.Accept (System.Linq.Expressions.ExpressionVisitor visitor) (at <61774763be294c9f8e2c781f10819224>:0)
System.Linq.Expressions.ExpressionVisitor.Visit (System.Linq.Expressions.Expression node) (at <61774763be294c9f8e2c781f10819224>:0)
Realms.RealmResultsVisitor.VisitMethodCall (System.Linq.Expressions.MethodCallExpression node) (at <5ee71c2d3038455ca732a3febcca3879>:0)
System.Linq.Expressions.MethodCallExpression.Accept (System.Linq.Expressions.ExpressionVisitor visitor) (at <61774763be294c9f8e2c781f10819224>:0)
System.Linq.Expressions.ExpressionVisitor.Visit (System.Linq.Expressions.Expression node) (at <61774763be294c9f8e2c781f10819224>:0)
Realms.RealmResults`1[T].GetOrCreateHandle () (at <5ee71c2d3038455ca732a3febcca3879>:0)
System.Lazy`1[T].ViaFactory (System.Threading.LazyThreadSafetyMode mode) (at <00c558282d074245ab3496e2d108079b>:0)
System.Lazy`1[T].ExecutionAndPublication (System.LazyHelper executionAndPublication, System.Boolean useDefaultConstructor) (at <00c558282d074245ab3496e2d108079b>:0)
System.Lazy`1[T].CreateValue () (at <00c558282d074245ab3496e2d108079b>:0)
System.Lazy`1[T].get_Value () (at <00c558282d074245ab3496e2d108079b>:0)
Realms.RealmCollectionBase`1+Enumerator[T]..ctor (Realms.RealmCollectionBase`1[T] parent) (at <5ee71c2d3038455ca732a3febcca3879>:0)
Realms.RealmCollectionBase`1[T].GetEnumerator () (at <5ee71c2d3038455ca732a3febcca3879>:0)
RealmExample.QueryTags () (at Assets/RealmExample.cs:32)

// Added the asterisk to match the behaviour of Contains()
string substring = "*T*";
IQueryable<Tag> tags = realm.All<Tag>()
                            // This works fine
                            .Where(t => t.Name.Like(substring, false));