How do I buy Bitcoins with Changelly or any other similar exchange boards?

If you are willing to use Changelly to buy cryptos then I would say you should use CoinSwitch .co instead. CoinSwitch. is the world’s largest cryptocurrency exchange aggregator which supports 300+ coins and over 45,000+ pairs. It provides an easier way of trading through the global exchanges like KuCoin, IDEX, Changelly, Bittrex, Cryptopia, ChangeNow and Changer without creating an account on them. You can exchange the coin from the exchange of your choice via the CoinSwitch.

So you will always get the best rates for your exchange from CoinSwitch. Give it a try!