https://developers.cloudflare.com/fundamentals/get-started/reference/http-request-headers/

try read CF-Connecting-IP or HTTPXFORWARDED_FOR headers

 public static string GetIpAddress(this IHttpContextAccessor accessor)
    {
        if (!string.IsNullOrEmpty(accessor.HttpContext.Request.Headers["CF-CONNECTING-IP"]))
            return accessor.HttpContext.Request.Headers["CF-CONNECTING-IP"];

        var ipAddress = accessor.HttpContext.GetServerVariable("HTTP_X_FORWARDED_FOR");

        if (!string.IsNullOrEmpty(ipAddress))
        {
            var addresses = ipAddress.Split(',');
            if (addresses.Length != 0)
                return addresses.Last();
        }

        return accessor.HttpContext.Connection.RemoteIpAddress.ToString();
    }