When People Openraise small coldcalling becomes more attractive compared to 3bets. It makes a Hugh difference if someone ist Opening 3x or 2,5 x (you can see this in snowie preflop app).

If someone minraises SB you should be able to defend almost every single Combo in the BB.

I would check the preflop advice of snowie from the button 2,5 x and add a few (at first suited) Hands.

In Position BU/CO I would expand my 3bet Range (around 7BB against 2.2 should work fine). I would be carefully to call more because you will get squeezed very often if aggressive Players are in the blinds.
Postflop: you Need to defend more of your range in general and implement a aggressive Check/Raise strategy against Rangebets on boards that favour the coldcaller. GTO+ will help you with constructing ranges.

"Underbets" isn't a standard term the way you're using it. The two situations you describe (preflop and postflop) are two different things. Also, a 2.2x raise size is very different from a 2.8x raise size. So let me disentangle the issues a bit here.

First, preflop. Theoretically speaking, as you open smaller, you can open wider. So you should expect a 2.2x raising range to be wider than a 2.8x raising range.

You can react to the raises in various ways.

In positions other than the BB: As Kamator123 mentioned above, it's dangerous to flat small raises if there are squeezers in the blinds. This is because the primary responsibility of defence against squeezes lies with the PFR (because he has the uncapped range), and if he's opening a wider range, he's going to have a tough time defending. So, if there are aggressive people in the blinds, you should look to 3-bet these raises to a small size to isolate and discourage squeezes.

In the BB, you can flat pretty wide because you're getting even better odds to flat (than if you have a 3x or 3.5x raise). You can do a lot of very interesting things. For instance, you can flat some really strong hands (like JJ or QQ) in the BB to help protect your really wide range, and make most of your EV by bluffcatching the continuation bets postflop. That said, the better play probably is to 3-bet pretty aggressively because Villain's range is relatively weaker.

Now postflop: A small bet (25% or 33% pot sizing) often indicates a range-betting strategy. A small bet on the flop just means that: "I have the range advantage on this flop. So I'm going to bet with all or most of my range so I can push equity against a weaker range, and you can't do much against it because the air in my range is protected by all the strong hands."

In general, you react against small c-bets by calling wider and raising aggressively. There are many ways to play and details will depend on the particular spot.

You can study these spots by node-locking a range-bet strategy in GTO+. Or if it's not a range bet, you can just add a 25% bet size option (plus whatever options people choose) and see how to react.