Use push-notifications to motivate players right when they need it. Here are notification principals. Every platform and game can have their own notification options. Use this document as a guided instruction.
Unity push notification
Choose your own notification plugin, as there are many variables and options.
iOS push notification
using (var req = new AuthorizationRequest(AuthorizationOption.AuthorizationOptionAlert | AuthorizationOption.AuthorizationOptionBadge, true))
{
  while (!req.IsFinished)
  {
    yield return null;
  };
  string res = "\n RequestAuthorization: \n";
  res += "\n finished: " + req.IsFinished;
  res += "\n granted :  " + req.Granted;
  res += "\n error:  " + req.Error;
  res += "\n deviceToken:  " + req.DeviceToken;
}
var timeTrigger = new iOSNotificationTimeIntervalTrigger()
{
  TimeInterval = new TimeSpan(0, minutes, seconds),
  Repeats = false
};
var notification = new iOSNotification()
{
  Identifier = "_game_reward_level_01"
  Title = "You unlocked new reward",
  Body = "You just unlocked new level of superhero",
  Subtitle = "Just reacheed level 25 and received option to unlock new reward, check it out",
  ShowInForeground = true,
  ForegroundPresentationOption = foregoungOption,
Unity push notification
iOS push notification
using (var req = new AuthorizationRequest(AuthorizationOption.AuthorizationOptionAlert | AuthorizationOption.AuthorizationOptionBadge, true)){Â Â while (!req.IsFinished)Â Â {Â Â Â Â yield return null;Â Â };
  string res = "\n RequestAuthorization: \n";  res += "\n finished: " + req.IsFinished;  res += "\n granted :  " + req.Granted;  res += "\n error:  " + req.Error;  res += "\n deviceToken:  " + req.DeviceToken;}
var timeTrigger = new iOSNotificationTimeIntervalTrigger(){  TimeInterval = new TimeSpan(0, minutes, seconds),  Repeats = false};
var notification = new iOSNotification(){  Identifier = "_game_reward_level_01"  Title = "You unlocked new reward",  Body = "You just unlocked new level of superhero",  Subtitle = "Just reacheed level 25 and received option to unlock new reward, check it out",  ShowInForeground = true,  ForegroundPresentationOption = foregoungOption,  CategoryIdentifier = "game_category_a",  ThreadIdentifier = thread,  Trigger = timeTrigger,};
iOSNotificationCenter.ScheduleNotification(notification);Android push notification
var c = new AndroidNotificationChannel(){  Id = "game_channel_id",  Name = "Game Channel",  Importance = Importance.High,  Description = "Generic notifications",};