How to rebind dropdown after RedirectToAction("Index") using ViewBag in ASP.NET Core MVC?

Getting issue during binding dropdown data to view using ViewBag. Getting Data properly and putting to the viewbag but it's not replacing the dropdown value on view.

Below is the Index page that binding with dropdown with viewbag-

Index.cshtml:

<div class="col-md-6">
    <label for="option1">System Role</label>
    <select id="rolelist" name="rolelist" class="form-control">
        <option value="">Select System Role</option>
        @foreach (var role in ViewBag.RoleList)
        {
            <option value="@role.roleId">@role.roleName</option>
        }
    </select>
</div>

And, this is the controller page where getting value from database and assigned to to viewbag on radio button click with different dropdown values and redirecting to the Index.cshtml page:

public async Task<IActionResult> Index()
{
        if (TempData["RoleCode"] != null)
        {
            string RoleCode = TempData["RoleCode"].ToString();

            List<RoleModel> roleList = JsonConvert.DeserializeObject<List<RoleModel>>(await _commonService.GetSystemRole(TempData["RoleCode"].ToString())).ToList();
            ViewBag.RoleList = roleList;
        }
        else
        {
            List<object> roleList = JsonConvert.DeserializeObject<List<object>>(await _commonService.GetSystemRole("MAAD"));
            ViewBag.RoleList = roleList;
        }

        return View();
}

[HttpPost]
public async Task<IActionResult> HandleRadioButton(string access)
{
        string RoleCode = string.Empty;

        if (access == "MasterAccess")
        {
            RoleCode = "MAAD";
        }
        else if (access == "StudyAccess")
        {
            RoleCode = "";
        }

        TempData["RoleCode"] = RoleCode;

        return RedirectToAction("Index");
}

Since you are trying with ajax,you have to redirect to target url or update dom youreslf in js/jquery codes

A minimal example based on your design:

public class MyController : Controller
    {

        private List<Role> Roles= Enumerable.Range(1, 5).Select(x => new Role
        {
            roleId = x,
            roleName=String.Format("RoleName{0}",x.ToString())
        }).ToList();
            
        public IActionResult Index()
        {
            var access = TempData["access"]?.ToString()??"";
            if (access == "odd")
            {
                Roles = Roles.Where(x => x.roleId % 2 != 0).ToList();
            }
             if (access == "even")
            {
                Roles = Roles.Where(x => x.roleId % 2 == 0).ToList();
            }
            ViewBag.RoleList=Roles;
            return View();
        }

        public IActionResult DropDown(string access)
        {
            TempData["access"] = access;
            return Ok();
            
        }

View：

<div id="mydropdownpartial">
     <partial name="DropDown"></partial>
</div>
<div>
  <lable>Odd</lable>
    <input  type="radio" name="sel" value="odd" />
    <lable>Even</lable>
<input  type="radio" name="sel" value="even" />
</div>

<script src="~/lib/jquery/dist/jquery.min.js"></script>
<script>    
    $(":radio").click(function () {
        var access=$(this).val()
        
        $.ajax({
            url: "DropDown?access="+access,
            success:function(response){
                //refresh your page here
                window.location.href="Index"
            }
        })
    })
</script>

Partial:

<div class="col-md-6">
    <label for="option1">System Role</label>
    <select id="rolelist" name="rolelist" class="form-control">
        <option value="">Select System Role</option>
        @foreach (var role in (ViewBag.RoleList as List<Role>))
        {
            <option value="@role.roleId">@role.roleName</option>
        }
    </select>
</div>

Result:

If you just want to refresh your dropdown,you could try as below:

public class MyController : Controller
    {

        private List<Role> Roles= Enumerable.Range(1, 5).Select(x => new Role
        {
            roleId = x,
            roleName=String.Format("RoleName{0}",x.ToString())
        }).ToList();
            
        public IActionResult Index()
        {
            
            ViewBag.RoleList=Roles;
            return View();
        }

        public IActionResult DropDown(string access)
        {
            
            if (access == "odd")
            {
                Roles = Roles.Where(x => x.roleId % 2 != 0).ToList();
            }
            else if (access == "even")
            {
                Roles = Roles.Where(x => x.roleId % 2 == 0).ToList();
            }

            ViewBag.RoleList = Roles;


            return PartialView();
        }
    }

jquery codes:

$(":radio").click(function () {
        var access=$(this).val()
        console.log(access)
        $.ajax({
            url: "DropDown?access="+access,
            success:function(response){
                $("#mydropdownpartial").html(response)
                
            }
        })
    })

Result:

You could see that the radio button keeps the selected value