In my opinion, cryptocurrencies are valued based on a simple formula, Coinmarketcap = Circulating Supply X Coin Price.
If the value of a crypto is its market cap, then it's by definition impossible for it to be over- or undervalued.
So, how can we decide if it's time to buy or to sell?

You can determine when to buy or sell through the simple equation, ie the circulating supply multiply by price over a period of time.
This situation fluctuate and that is the more reason you thave to study some it charts to determine the trading partern of the buying and selling. I think there other better ways of valuating crypto, I'm ready to learn if you bring them on the forum. Thank you .
My point was only that establishing the Market Cap of a coin doesn't mean establishing its value.
The MC of a coin at a given time is not a criterion to decide if buy or sell that coin, just as the price of a coin is not a criterion to decide weather to buy or sell.
You must use other criteria, as for example comparing MC and price of a coin in different times. Analysing MC and price movements.
There comes chart analysis to play.
Another way to evaluate a crypto is to look at its potential applications in the real world, in the economy.
Another one is to look at which investors or institutional users are interested in it.