The best advice I can give is do as much serve and receive drills as you can. You get a bucket of balls. You get a friend. You serve underspin. He pushes back. Then you loop his push and play out the point. You finish a bucket and then switch so that he is serving and you are pushing.
Here are examples that are more specific:
1) First player serves underspin to receiver's backhand.
2) Second person pushes to the server's forehand side.
3) First person tries to loop the underspin.
4) Play out the point.
Obviously the serve can go to the forehand or the backhand side depending on if the receiver wants to work on pushing with the forehand or backhand. The push can also go to the forehand or backhand side for similar reasons but also if you want to work on using footwork to loop with the forehand from the backhand side, or banana loop with the backhand from the forehand side like Zhang Jike.
When you are first learning to loop underspin it is worth making it so that the push is coming to the same basic place over and over again so you work on that shot without worrying about where the ball will go on the return of serve. Once you are solid with looping short balls from the forehand and backhand, then it is time to do serve and receive drills with the random element of not knowing where the push is going.
By doing these kinds of drills you learn how to loop underspin and if you can do that, you can loop topspin. You also are forced to improve your strokes because you need decent mechanics to loop short, low underspin. When you practice this a lot, and then you start doing topspin rallies you start realizing how much more power and spin you get from your strokes just from practicing looping underspin.
By the way, it sounds like the equipment you are using should be fine for developing these skills and if you just keep practicing trying to loop underspin you will get the technique eventually. When the ball starts going on the table with good spin consistently you are doing something right.
If you know someone who has long pips and is a good blocker and chopper, hitting with them and practicing looping while they chop is really good for these skills as well. You practicing returning their shots with topspin and trying to get as much spin on the ball as you can rather than just smashing it which can work but does not help you develop the skill or control of looping or 3rd ball attacks.